hypertext transfer protocol version 2 (http/2)...hypertext transfer protocol version 2 (http/2)...

Internet Engineering Task Force (IETF) M. BelsheRequest for Comments: 7540 BitGoCategory: Standards Track R. PeonISSN: 2070-1721 Google, Inc

M. Thomson, EditorMozilla

May 2015

Hypertext Transfer Protocol Version 2 (HTTP/2)

Abstract

This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol(HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resourcesand a reduced perception of latency by introducing header field compression and allowing multiple concurrentexchanges on the same connection. It also introduces unsolicited push of representations from servers to clients.

This specification is an alternative to, but does not obsolete, the HTTP/1.1 message syntax. HTTP's existingsemantics remain unchanged.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus ofthe IETF community. It has received public review and has been approved for publication by the InternetEngineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC5741.

Information about the current status of this document, any errata, and how to provide feedback on it may beobtained at http://www.rfc-editor.org/info/rfc75401.

Copyright Notice

Copyright © 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents(http://trustee.ietf.org/license-info2) in effect on the date of publication of this document. Please reviewthese documents carefully, as they describe your rights and restrictions with respect to this document. CodeComponents extracted from this document must include Simplified BSD License text as described in Section4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSDLicense.

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RFC 7540 HTTP/2 May 2015

Table of Contents

1 Introduction...............................................................................................................................................................5

2 HTTP/2 Protocol Overview..................................................................................................................................... 6

2.1 Document Organization.........................................................................................................................................6

2.2 Conventions and Terminology.............................................................................................................................. 6

3 Starting HTTP/2....................................................................................................................................................... 8

3.1 HTTP/2 Version Identification..............................................................................................................................8

3.2 Starting HTTP/2 for "http" URIs.......................................................................................................................... 8 3.2.1 HTTP2-Settings Header Field.......................................................................................................................... 9

3.3 Starting HTTP/2 for "https" URIs........................................................................................................................ 9

3.4 Starting HTTP/2 with Prior Knowledge............................................................................................................. 10

3.5 HTTP/2 Connection Preface............................................................................................................................... 10

4 HTTP Frames..........................................................................................................................................................12

4.1 Frame Format.......................................................................................................................................................12

4.2 Frame Size........................................................................................................................................................... 12

4.3 Header Compression and Decompression...........................................................................................................13

5 Streams and Multiplexing......................................................................................................................................14

5.1 Stream States....................................................................................................................................................... 14 5.1.1 Stream Identifiers............................................................................................................................................17 5.1.2 Stream Concurrency....................................................................................................................................... 18

5.2 Flow Control........................................................................................................................................................18 5.2.1 Flow-Control Principles..................................................................................................................................18 5.2.2 Appropriate Use of Flow Control.................................................................................................................. 19

5.3 Stream Priority.....................................................................................................................................................19 5.3.1 Stream Dependencies......................................................................................................................................20 5.3.2 Dependency Weighting.................................................................................................................................. 20 5.3.3 Reprioritization............................................................................................................................................... 21 5.3.4 Prioritization State Management.................................................................................................................... 21 5.3.5 Default Priorities.............................................................................................................................................22

5.4 Error Handling..................................................................................................................................................... 22 5.4.1 Connection Error Handling............................................................................................................................ 22 5.4.2 Stream Error Handling................................................................................................................................... 22 5.4.3 Connection Termination................................................................................................................................. 23

5.5 Extending HTTP/2...............................................................................................................................................23

6 Frame Definitions................................................................................................................................................... 24

6.1 DATA...................................................................................................................................................................24

6.2 HEADERS........................................................................................................................................................... 25

6.3 PRIORITY........................................................................................................................................................... 26

6.4 RST_STREAM.................................................................................................................................................... 26

6.5 SETTINGS...........................................................................................................................................................27

Belshe, et al. Standards Track []


6.5.1 SETTINGS Format.........................................................................................................................................28 6.5.2 Defined SETTINGS Parameters.....................................................................................................................28 6.5.3 Settings Synchronization................................................................................................................................ 29

6.6 PUSH_PROMISE................................................................................................................................................ 29

6.7 PING.................................................................................................................................................................... 30

6.8 GOAWAY........................................................................................................................................................... 31

6.9 WINDOW_UPDATE.......................................................................................................................................... 33 6.9.1 The Flow-Control Window............................................................................................................................ 33 6.9.2 Initial Flow-Control Window Size.................................................................................................................34 6.9.3 Reducing the Stream Window Size............................................................................................................... 35

6.10 CONTINUATION............................................................................................................................................. 35

7 Error Codes.............................................................................................................................................................36

8 HTTP Message Exchanges.................................................................................................................................... 37

8.1 HTTP Request/Response Exchange.................................................................................................................... 37 8.1.1 Upgrading from HTTP/2................................................................................................................................ 38 8.1.2 HTTP Header Fields.......................................................................................................................................38 8.1.3 Examples.........................................................................................................................................................40 8.1.4 Request Reliability Mechanisms in HTTP/2................................................................................................. 42

8.2 Server Push.......................................................................................................................................................... 42 8.2.1 Push Requests................................................................................................................................................. 43 8.2.2 Push Responses...............................................................................................................................................44

8.3 The CONNECT Method..................................................................................................................................... 44

9 Additional HTTP Requirements/Considerations................................................................................................ 46

9.1 Connection Management..................................................................................................................................... 46 9.1.1 Connection Reuse........................................................................................................................................... 46 9.1.2 The 421 (Misdirected Request) Status Code................................................................................................. 46

9.2 Use of TLS Features........................................................................................................................................... 47 9.2.1 TLS 1.2 Features............................................................................................................................................ 47 9.2.2 TLS 1.2 Cipher Suites....................................................................................................................................48

10 Security Considerations....................................................................................................................................... 49

10.1 Server Authority................................................................................................................................................ 49

10.2 Cross-Protocol Attacks...................................................................................................................................... 49

10.3 Intermediary Encapsulation Attacks..................................................................................................................49

10.4 Cacheability of Pushed Responses....................................................................................................................49

10.5 Denial-of-Service Considerations......................................................................................................................49 10.5.1 Limits on Header Block Size....................................................................................................................... 50 10.5.2 CONNECT Issues.........................................................................................................................................50

10.6 Use of Compression.......................................................................................................................................... 51

10.7 Use of Padding.................................................................................................................................................. 51

10.8 Privacy Considerations...................................................................................................................................... 51

11 IANA Considerations........................................................................................................................................... 53



11.1 Registration of HTTP/2 Identification Strings..................................................................................................53

11.2 Frame Type Registry......................................................................................................................................... 53

11.3 Settings Registry................................................................................................................................................54

11.4 Error Code Registry...........................................................................................................................................54

11.5 HTTP2-Settings Header Field Registration...................................................................................................... 55

11.6 PRI Method Registration...................................................................................................................................55

11.7 The 421 (Misdirected Request) HTTP Status Code......................................................................................... 55

11.8 The h2c Upgrade Token....................................................................................................................................55

12 References.............................................................................................................................................................. 57

12.1 Normative References....................................................................................................................................... 57

12.2 Informative References......................................................................................................................................58

Appendix A TLS 1.2 Cipher Suite Black List........................................................................................................59

Authors' Addresses.....................................................................................................................................................67



1. Introduction

The Hypertext Transfer Protocol (HTTP) is a wildly successful protocol. However, the way HTTP/1.1 uses theunderlying transport ([RFC7230], Section 6) has several characteristics that have a negative overall effect onapplication performance today.

In particular, HTTP/1.0 allowed only one request to be outstanding at a time on a given TCP connection.HTTP/1.1 added request pipelining, but this only partially addressed request concurrency and still suffersfrom head-of-line blocking. Therefore, HTTP/1.0 and HTTP/1.1 clients that need to make many requests usemultiple connections to a server in order to achieve concurrency and thereby reduce latency.

Furthermore, HTTP header fields are often repetitive and verbose, causing unnecessary network traffic as wellas causing the initial TCP [TCP] congestion window to quickly fill. This can result in excessive latency whenmultiple requests are made on a new TCP connection.

HTTP/2 addresses these issues by defining an optimized mapping of HTTP's semantics to an underlyingconnection. Specifically, it allows interleaving of request and response messages on the same connection anduses an efficient coding for HTTP header fields. It also allows prioritization of requests, letting more importantrequests complete more quickly, further improving performance.

The resulting protocol is more friendly to the network because fewer TCP connections can be used incomparison to HTTP/1.x. This means less competition with other flows and longer-lived connections, which inturn lead to better utilization of available network capacity.

Finally, HTTP/2 also enables more efficient processing of messages through use of binary message framing.



2. HTTP/2 Protocol Overview

HTTP/2 provides an optimized transport for HTTP semantics. HTTP/2 supports all of the core features ofHTTP/1.1 but aims to be more efficient in several ways.

The basic protocol unit in HTTP/2 is a frame (Section 4.1). Each frame type serves a different purpose. Forexample, HEADERS and DATA frames form the basis of HTTP requests and responses (Section 8.1); otherframe types like SETTINGS, WINDOW_UPDATE, and PUSH_PROMISE are used in support of otherHTTP/2 features.

Multiplexing of requests is achieved by having each HTTP request/response exchange associated with its ownstream (Section 5). Streams are largely independent of each other, so a blocked or stalled request or responsedoes not prevent progress on other streams.

Flow control and prioritization ensure that it is possible to efficiently use multiplexed streams. Flow control(Section 5.2) helps to ensure that only data that can be used by a receiver is transmitted. Prioritization(Section 5.3) ensures that limited resources can be directed to the most important streams first.

HTTP/2 adds a new interaction mode whereby a server can push responses to a client (Section 8.2). Serverpush allows a server to speculatively send data to a client that the server anticipates the client will need, tradingoff some network usage against a potential latency gain. The server does this by synthesizing a request, whichit sends as a PUSH_PROMISE frame. The server is then able to send a response to the synthetic request on aseparate stream.

Because HTTP header fields used in a connection can contain large amounts of redundant data, frames thatcontain them are compressed (Section 4.3). This has especially advantageous impact upon request sizes in thecommon case, allowing many requests to be compressed into one packet.

2.1. Document Organization

The HTTP/2 specification is split into four parts:

• Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is initiated.

• The frame (Section 4) and stream (Section 5) layers describe the way HTTP/2 frames are structured andformed into multiplexed streams.

• Frame (Section 6) and error (Section 7) definitions include details of the frame and error types used inHTTP/2.

• HTTP mappings (Section 8) and additional requirements (Section 9) describe how HTTP semantics areexpressed using frames and streams.

While some of the frame and stream layer concepts are isolated from HTTP, this specification does not define acompletely generic frame layer. The frame and stream layers are tailored to the needs of the HTTP protocol andserver push.

2.2. Conventions and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULDNOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described inRFC 2119 [RFC2119].

All numeric values are in network byte order. Values are unsigned unless otherwise indicated. Literal valuesare provided in decimal or hexadecimal as appropriate. Hexadecimal literals are prefixed with 0x to distinguishthem from decimal literals.

The following terms are used:

client: The endpoint that initiates an HTTP/2 connection. Clients send HTTPrequests and receive HTTP responses.

connection: A transport-layer connection between two endpoints.



connection error: An error that affects the entire HTTP/2 connection.

endpoint: Either the client or server of the connection.

frame: The smallest unit of communication within an HTTP/2 connection, consistingof a header and a variable-length sequence of octets structured according tothe frame type.

peer: An endpoint. When discussing a particular endpoint, "peer" refers to theendpoint that is remote to the primary subject of discussion.

receiver: An endpoint that is receiving frames.

sender: An endpoint that is transmitting frames.

server: The endpoint that accepts an HTTP/2 connection. Servers receive HTTPrequests and send HTTP responses.

stream: A bidirectional flow of frames within the HTTP/2 connection.

stream error: An error on the individual HTTP/2 stream.

Finally, the terms "gateway", "intermediary", "proxy", and "tunnel" are defined in Section 2.3 of [RFC7230].Intermediaries act as both client and server at different times.

The term "payload body" is defined in Section 3.3 of [RFC7230].



3. Starting HTTP/2

An HTTP/2 connection is an application-layer protocol running on top of a TCP connection ([TCP]). The clientis the TCP connection initiator.

HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1. HTTP/2 shares the same default portnumbers: 80 for "http" URIs and 443 for "https" URIs. As a result, implementations processing requests fortarget resource URIs like http://example.org/foo or https://example.com/bar are requiredto first discover whether the upstream server (the immediate peer to which the client wishes to establish aconnection) supports HTTP/2.

The means by which support for HTTP/2 is determined is different for "http" and "https" URIs. Discovery for"http" URIs is described in Section 3.2. Discovery for "https" URIs is described in Section 3.3.

3.1. HTTP/2 Version Identification

The protocol defined in this document has two identifiers.

• The string "h2" identifies the protocol where HTTP/2 uses Transport Layer Security (TLS) [TLS12]. Thisidentifier is used in the TLS application-layer protocol negotiation (ALPN) extension [TLS-ALPN] fieldand in any place where HTTP/2 over TLS is identified.

The "h2" string is serialized into an ALPN protocol identifier as the two-octet sequence: 0x68, 0x32.

• The string "h2c" identifies the protocol where HTTP/2 is run over cleartext TCP. This identifier is used inthe HTTP/1.1 Upgrade header field and in any place where HTTP/2 over TCP is identified.

The "h2c" string is reserved from the ALPN identifier space but describes a protocol that does not use TLS.

Negotiating "h2" or "h2c" implies the use of the transport, security, framing, and message semantics describedin this document.

3.2. Starting HTTP/2 for "http" URIs

A client that makes a request for an "http" URI without prior knowledge about support for HTTP/2 on thenext hop uses the HTTP Upgrade mechanism (Section 6.7 of [RFC7230]). The client does so by making anHTTP/1.1 request that includes an Upgrade header field with the "h2c" token. Such an HTTP/1.1 requestMUST include exactly one HTTP2-Settings (Section 3.2.1) header field.

For example:

GET / HTTP/1.1Host: server.example.comConnection: Upgrade, HTTP2-SettingsUpgrade: h2cHTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>

Requests that contain a payload body MUST be sent in their entirety before the client can send HTTP/2 frames.This means that a large request can block the use of the connection until it is completely sent.

If concurrency of an initial request with subsequent requests is important, an OPTIONS request can be used toperform the upgrade to HTTP/2, at the cost of an additional round trip.

A server that does not support HTTP/2 can respond to the request as though the Upgrade header field wereabsent:



HTTP/1.1 200 OKContent-Length: 243Content-Type: text/html

...

A server MUST ignore an "h2" token in an Upgrade header field. Presence of a token with "h2" impliesHTTP/2 over TLS, which is instead negotiated as described in Section 3.3.

A server that supports HTTP/2 accepts the upgrade with a 101 (Switching Protocols) response. After the emptyline that terminates the 101 response, the server can begin sending HTTP/2 frames. These frames MUSTinclude a response to the request that initiated the upgrade.

For example:

HTTP/1.1 101 Switching ProtocolsConnection: UpgradeUpgrade: h2c

[ HTTP/2 connection ...

The first HTTP/2 frame sent by the server MUST be a server connection preface (Section 3.5) consisting of aSETTINGS frame (Section 6.5). Upon receiving the 101 response, the client MUST send a connection preface(Section 3.5), which includes a SETTINGS frame.

The HTTP/1.1 request that is sent prior to upgrade is assigned a stream identifier of 1 (see Section 5.1.1) withdefault priority values (Section 5.3.5). Stream 1 is implicitly "half-closed" from the client toward the server(see Section 5.1), since the request is completed as an HTTP/1.1 request. After commencing the HTTP/2connection, stream 1 is used for the response.

3.2.1. HTTP2-Settings Header Field

A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly one HTTP2-Settings headerfield. The HTTP2-Settings header field is a connection-specific header field that includes parameters thatgovern the HTTP/2 connection, provided in anticipation of the server accepting the request to upgrade.

HTTP2-Settings = token68

A server MUST NOT upgrade the connection to HTTP/2 if this header field is not present or if more than oneis present. A server MUST NOT send this header field.

The content of the HTTP2-Settings header field is the payload of a SETTINGS frame (Section 6.5),encoded as a base64url string (that is, the URL- and filename-safe Base64 encoding described in Section 5of [RFC4648], with any trailing '=' characters omitted). The ABNF [RFC5234] production for token68 isdefined in Section 2.1 of [RFC7235].

Since the upgrade is only intended to apply to the immediate connection, a client sending the HTTP2-Settings header field MUST also send HTTP2-Settings as a connection option in the Connectionheader field to prevent it from being forwarded (see Section 6.1 of [RFC7230]).

A server decodes and interprets these values as it would any other SETTINGS frame. Explicitacknowledgement of these settings (Section 6.5.3) is not necessary, since a 101 response serves as implicitacknowledgement. Providing these values in the upgrade request gives a client an opportunity to provideparameters prior to receiving any frames from the server.



3.3. Starting HTTP/2 for "https" URIs

A client that makes a request to an "https" URI uses TLS [TLS12] with the application-layer protocolnegotiation (ALPN) extension [TLS-ALPN].

HTTP/2 over TLS uses the "h2" protocol identifier. The "h2c" protocol identifier MUST NOT be sent by aclient or selected by a server; the "h2c" protocol identifier describes a protocol that does not use TLS.

Once TLS negotiation is complete, both the client and the server MUST send a connection preface(Section 3.5).

3.4. Starting HTTP/2 with Prior Knowledge

A client can learn that a particular server supports HTTP/2 by other means. For example, [ALT-SVC] describesa mechanism for advertising this capability.

A client MUST send the connection preface (Section 3.5) and then MAY immediately send HTTP/2 framesto such a server; servers can identify these connections by the presence of the connection preface. This onlyaffects the establishment of HTTP/2 connections over cleartext TCP; implementations that support HTTP/2over TLS MUST use protocol negotiation in TLS [TLS-ALPN].

Likewise, the server MUST send a connection preface (Section 3.5).

Without additional information, prior support for HTTP/2 is not a strong signal that a given server willsupport HTTP/2 for future connections. For example, it is possible for server configurations to change, forconfigurations to differ between instances in clustered servers, or for network conditions to change.

3.5. HTTP/2 Connection Preface

In HTTP/2, each endpoint is required to send a connection preface as a final confirmation of the protocol inuse and to establish the initial settings for the HTTP/2 connection. The client and server each send a differentconnection preface.

The client connection preface starts with a sequence of 24 octets, which in hex notation is:

0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a

That is, the connection preface starts with the string PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n). Thissequence MUST be followed by a SETTINGS frame (Section 6.5), which MAY be empty. The client sendsthe client connection preface immediately upon receipt of a 101 (Switching Protocols) response (indicatinga successful upgrade) or as the first application data octets of a TLS connection. If starting an HTTP/2connection with prior knowledge of server support for the protocol, the client connection preface is sent uponconnection establishment.

Note: The client connection preface is selected so that a large proportion of HTTP/1.1 or HTTP/1.0servers and intermediaries do not attempt to process further frames. Note that this does not address theconcerns raised in [TALKING].

The server connection preface consists of a potentially empty SETTINGS frame (Section 6.5) that MUST bethe first frame the server sends in the HTTP/2 connection.

The SETTINGS frames received from a peer as part of the connection preface MUST be acknowledged (seeSection 6.5.3) after sending the connection preface.

To avoid unnecessary latency, clients are permitted to send additional frames to the server immediatelyafter sending the client connection preface, without waiting to receive the server connection preface. It isimportant to note, however, that the server connection preface SETTINGS frame might include parametersthat necessarily alter how a client is expected to communicate with the server. Upon receiving the SETTINGSframe, the client is expected to honor any parameters established. In some configurations, it is possible for theserver to transmit SETTINGS before the client sends additional frames, providing an opportunity to avoid thisissue.



Clients and servers MUST treat an invalid connection preface as a connection error (Section 5.4.1) of typePROTOCOL_ERROR. A GOAWAY frame (Section 6.8) MAY be omitted in this case, since an invalidpreface indicates that the peer is not using HTTP/2.



4. HTTP Frames

Once the HTTP/2 connection is established, endpoints can begin exchanging frames.

4.1. Frame Format

All frames begin with a fixed 9-octet header followed by a variable-length payload.

+-----------------------------------------------+ | Length (24) | +---------------+---------------+---------------+ | Type (8) | Flags (8) | +-+-------------+---------------+-------------------------------+ |R| Stream Identifier (31) | +=+=============================================================+ | Frame Payload (0...) ... +---------------------------------------------------------------+

Figure 1: Frame Layout

The fields of the frame header are defined as:

Length:The length of the frame payload expressed as an unsigned 24-bit integer. Values greater than 214 (16,384)MUST NOT be sent unless the receiver has set a larger value for SETTINGS_MAX_FRAME_SIZE.

The 9 octets of the frame header are not included in this value.

Type:The 8-bit type of the frame. The frame type determines the format and semantics of the frame. ImplementationsMUST ignore and discard any frame that has a type that is unknown.

Flags:An 8-bit field reserved for boolean flags specific to the frame type.

Flags are assigned semantics specific to the indicated frame type. Flags that have no defined semantics for aparticular frame type MUST be ignored and MUST be left unset (0x0) when sending.

R:A reserved 1-bit field. The semantics of this bit are undefined, and the bit MUST remain unset (0x0) whensending and MUST be ignored when receiving.

StreamIdentifier:A stream identifier (see Section 5.1.1) expressed as an unsigned 31-bit integer. The value 0x0 is reserved forframes that are associated with the connection as a whole as opposed to an individual stream.

The structure and content of the frame payload is dependent entirely on the frame type.

4.2. Frame Size

The size of a frame payload is limited by the maximum size that a receiver advertises in theSETTINGS_MAX_FRAME_SIZE setting. This setting can have any value between 214 (16,384) and 224-1(16,777,215) octets, inclusive.

All implementations MUST be capable of receiving and minimally processing frames up to 214 octets in length,plus the 9-octet frame header (Section 4.1). The size of the frame header is not included when describing framesizes.

Note: Certain frame types, such as PING (Section 6.7), impose additional limits on the amount of payloaddata allowed.

An endpoint MUST send an error code of FRAME_SIZE_ERROR if a frame exceeds the size defined inSETTINGS_MAX_FRAME_SIZE, exceeds any limit defined for the frame type, or is too small to containmandatory frame data. A frame size error in a frame that could alter the state of the entire connection MUST betreated as a connection error (Section 5.4.1); this includes any frame carrying a header block (Section 4.3) (thatis, HEADERS, PUSH_PROMISE, and CONTINUATION), SETTINGS, and any frame with a stream identifierof 0.



Endpoints are not obligated to use all available space in a frame. Responsiveness can be improved by usingframes that are smaller than the permitted maximum size. Sending large frames can result in delays in sendingtime-sensitive frames (such as RST_STREAM, WINDOW_UPDATE, or PRIORITY), which, if blocked by thetransmission of a large frame, could affect performance.

4.3. Header Compression and Decompression

Just as in HTTP/1, a header field in HTTP/2 is a name with one or more associated values. Header fields areused within HTTP request and response messages as well as in server push operations (see Section 8.2).

Header lists are collections of zero or more header fields. When transmitted over a connection, a header listis serialized into a header block using HTTP header compression [COMPRESSION]. The serialized headerblock is then divided into one or more octet sequences, called header block fragments, and transmitted withinthe payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6), or CONTINUATION (Section 6.10)frames.

The Cookie header field [COOKIE] is treated specially by the HTTP mapping (see Section 8.1.2.5).

A receiving endpoint reassembles the header block by concatenating its fragments and then decompresses theblock to reconstruct the header list.

A complete header block consists of either:

• a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag set, or

• a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag cleared and one or moreCONTINUATION frames, where the last CONTINUATION frame has the END_HEADERS flag set.

Header compression is stateful. One compression context and one decompression context are used for the entireconnection. A decoding error in a header block MUST be treated as a connection error (Section 5.4.1) of typeCOMPRESSION_ERROR.

Each header block is processed as a discrete unit. Header blocks MUST be transmitted as a contiguoussequence of frames, with no interleaved frames of any other type or from any other stream. The last frame ina sequence of HEADERS or CONTINUATION frames has the END_HEADERS flag set. The last frame in asequence of PUSH_PROMISE or CONTINUATION frames has the END_HEADERS flag set. This allows aheader block to be logically equivalent to a single frame.

Header block fragments can only be sent as the payload of HEADERS, PUSH_PROMISE, orCONTINUATION frames because these frames carry data that can modify the compression context maintainedby a receiver. An endpoint receiving HEADERS, PUSH_PROMISE, or CONTINUATION frames needs toreassemble header blocks and perform decompression even if the frames are to be discarded. A receiver MUSTterminate the connection with a connection error (Section 5.4.1) of type COMPRESSION_ERROR if it doesnot decompress a header block.



5. Streams and Multiplexing

A "stream" is an independent, bidirectional sequence of frames exchanged between the client and server withinan HTTP/2 connection. Streams have several important characteristics:

• A single HTTP/2 connection can contain multiple concurrently open streams, with either endpointinterleaving frames from multiple streams.

• Streams can be established and used unilaterally or shared by either the client or server.

• Streams can be closed by either endpoint.

• The order in which frames are sent on a stream is significant. Recipients process frames in the order theyare received. In particular, the order of HEADERS and DATA frames is semantically significant.

• Streams are identified by an integer. Stream identifiers are assigned to streams by the endpoint initiating thestream.

5.1. Stream States

The lifecycle of a stream is shown in Figure 2.



+--------+ send PP | | recv PP ,--------| idle |--------. / | | \ v +--------+ v +----------+ | +----------+ | | | send H / | | ,------| reserved | | recv H | reserved |------. | | (local) | | | (remote) | | | +----------+ v +----------+ | | | +--------+ | | | | recv ES | | send ES | | | send H | ,-------| open |-------. | recv H | | | / | | \ | | | v v +--------+ v v | | +----------+ | +----------+ | | | half | | | half | | | | closed | | send R / | closed | | | | (remote) | | recv R | (local) | | | +----------+ | +----------+ | | | | | | | | send ES / | recv ES / | | | | send R / v send R / | | | | recv R +--------+ recv R | | | send R / `----------->| |<-----------' send R / | | recv R | closed | recv R | `----------------------->| |<----------------------' +--------+

send: endpoint sends this frame recv: endpoint receives this frame

H: HEADERS frame (with implied CONTINUATIONs) PP: PUSH_PROMISE frame (with implied CONTINUATIONs) ES: END_STREAM flag R: RST_STREAM frame

Figure 2: Stream States

Note that this diagram shows stream state transitions and the frames and flags that affect those transitionsonly. In this regard, CONTINUATION frames do not result in state transitions; they are effectively part of theHEADERS or PUSH_PROMISE that they follow. For the purpose of state transitions, the END_STREAM flagis processed as a separate event to the frame that bears it; a HEADERS frame with the END_STREAM flag setcan cause two state transitions.

Both endpoints have a subjective view of the state of a stream that could be different when frames are in transit.Endpoints do not coordinate the creation of streams; they are created unilaterally by either endpoint. Thenegative consequences of a mismatch in states are limited to the "closed" state after sending RST_STREAM,where frames might be received for some time after closing.

Streams have the following states:

idle:All streams start in the "idle" state.

The following transitions are valid from this state:



• Sending or receiving a HEADERS frame causes the stream to become "open". The stream identifier isselected as described in Section 5.1.1. The same HEADERS frame can also cause a stream to immediatelybecome "half-closed".

• Sending a PUSH_PROMISE frame on another stream reserves the idle stream that is identified for later use.The stream state for the reserved stream transitions to "reserved (local)".

• Receiving a PUSH_PROMISE frame on another stream reserves an idle stream that is identified for lateruse. The stream state for the reserved stream transitions to "reserved (remote)".

• Note that the PUSH_PROMISE frame is not sent on the idle stream but references the newly reservedstream in the Promised Stream ID field.

Receiving any frame other than HEADERS or PRIORITY on a stream in this state MUST be treated as aconnection error (Section 5.4.1) of type PROTOCOL_ERROR.

reserved(local):A stream in the "reserved (local)" state is one that has been promised by sending a PUSH_PROMISE frame.A PUSH_PROMISE frame reserves an idle stream by associating the stream with an open stream that wasinitiated by the remote peer (see Section 8.2).

In this state, only the following transitions are possible:

• The endpoint can send a HEADERS frame. This causes the stream to open in a "half-closed (remote)" state.

• Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases thestream reservation.

An endpoint MUST NOT send any type of frame other than HEADERS, RST_STREAM, or PRIORITY in thisstate.

A PRIORITY or WINDOW_UPDATE frame MAY be received in this state. Receiving any type of frame otherthan RST_STREAM, PRIORITY, or WINDOW_UPDATE on a stream in this state MUST be treated as aconnection error (Section 5.4.1) of type PROTOCOL_ERROR.

reserved(remote):A stream in the "reserved (remote)" state has been reserved by a remote peer.

In this state, only the following transitions are possible:

• Receiving a HEADERS frame causes the stream to transition to "half-closed (local)".

• Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases thestream reservation.

An endpoint MAY send a PRIORITY frame in this state to reprioritize the reserved stream. An endpointMUST NOT send any type of frame other than RST_STREAM, WINDOW_UPDATE, or PRIORITY in thisstate.

Receiving any type of frame other than HEADERS, RST_STREAM, or PRIORITY on a stream in this stateMUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

open:A stream in the "open" state may be used by both peers to send frames of any type. In this state, sending peersobserve advertised stream-level flow-control limits (Section 5.2).

From this state, either endpoint can send a frame with an END_STREAM flag set, which causes the stream totransition into one of the "half-closed" states. An endpoint sending an END_STREAM flag causes the streamstate to become "half-closed (local)"; an endpoint receiving an END_STREAM flag causes the stream state tobecome "half-closed (remote)".

Either endpoint can send a RST_STREAM frame from this state, causing it to transition immediately to"closed".

half-closed(local):

A stream that is in the "half-closed (local)" state cannot be used for sending frames other thanWINDOW_UPDATE, PRIORITY, and RST_STREAM.

A stream transitions from this state to "closed" when a frame that contains an END_STREAM flag is receivedor when either peer sends a RST_STREAM frame.



An endpoint can receive any type of frame in this state. Providing flow-control credit usingWINDOW_UPDATE frames is necessary to continue receiving flow-controlled frames. In this state, a receivercan ignore WINDOW_UPDATE frames, which might arrive for a short period after a frame bearing theEND_STREAM flag is sent.

PRIORITY frames received in this state are used to reprioritize streams that depend on the identified stream.

half-closed(remote):

A stream that is "half-closed (remote)" is no longer being used by the peer to send frames. In this state, anendpoint is no longer obligated to maintain a receiver flow-control window.

If an endpoint receives additional frames, other than WINDOW_UPDATE, PRIORITY, or RST_STREAM, fora stream that is in this state, it MUST respond with a stream error (Section 5.4.2) of type STREAM_CLOSED.

A stream that is "half-closed (remote)" can be used by the endpoint to send frames of any type. In this state, theendpoint continues to observe advertised stream-level flow-control limits (Section 5.2).

A stream can transition from this state to "closed" by sending a frame that contains an END_STREAM flag orwhen either peer sends a RST_STREAM frame.

closed:The "closed" state is the terminal state.

An endpoint MUST NOT send frames other than PRIORITY on a closed stream. An endpoint that receives anyframe other than PRIORITY after receiving a RST_STREAM MUST treat that as a stream error (Section 5.4.2)of type STREAM_CLOSED. Similarly, an endpoint that receives any frames after receiving a frame with theEND_STREAM flag set MUST treat that as a connection error (Section 5.4.1) of type STREAM_CLOSED,unless the frame is permitted as described below.

WINDOW_UPDATE or RST_STREAM frames can be received in this state for a short period after a DATAor HEADERS frame containing an END_STREAM flag is sent. Until the remote peer receives and processesRST_STREAM or the frame bearing the END_STREAM flag, it might send frames of these types. EndpointsMUST ignore WINDOW_UPDATE or RST_STREAM frames received in this state, though endpoints MAYchoose to treat frames that arrive a significant time after sending END_STREAM as a connection error(Section 5.4.1) of type PROTOCOL_ERROR.

PRIORITY frames can be sent on closed streams to prioritize streams that are dependent on the closed stream.Endpoints SHOULD process PRIORITY frames, though they can be ignored if the stream has been removedfrom the dependency tree (see Section 5.3.4).

If this state is reached as a result of sending a RST_STREAM frame, the peer that receives the RST_STREAMmight have already sent — or enqueued for sending — frames on the stream that cannot be withdrawn. Anendpoint MUST ignore frames that it receives on closed streams after it has sent a RST_STREAM frame. Anendpoint MAY choose to limit the period over which it ignores frames and treat frames that arrive after thistime as being in error.

Flow-controlled frames (i.e., DATA) received after sending RST_STREAM are counted toward the connectionflow-control window. Even though these frames might be ignored, because they are sent before the senderreceives the RST_STREAM, the sender will consider the frames to count against the flow-control window.

An endpoint might receive a PUSH_PROMISE frame after it sends RST_STREAM. PUSH_PROMISE causesa stream to become "reserved" even if the associated stream has been reset. Therefore, a RST_STREAM isneeded to close an unwanted promised stream.

In the absence of more specific guidance elsewhere in this document, implementations SHOULD treatthe receipt of a frame that is not expressly permitted in the description of a state as a connection error(Section 5.4.1) of type PROTOCOL_ERROR. Note that PRIORITY can be sent and received in any streamstate. Frames of unknown types are ignored.

An example of the state transitions for an HTTP request/response exchange can be found in Section 8.1. Anexample of the state transitions for server push can be found in Sections 8.2.1 and 8.2.2.



5.1.1. Stream Identifiers

Streams are identified with an unsigned 31-bit integer. Streams initiated by a client MUST use odd-numberedstream identifiers; those initiated by the server MUST use even-numbered stream identifiers. A streamidentifier of zero (0x0) is used for connection control messages; the stream identifier of zero cannot be used toestablish a new stream.

HTTP/1.1 requests that are upgraded to HTTP/2 (see Section 3.2) are responded to with a stream identifier ofone (0x1). After the upgrade completes, stream 0x1 is "half-closed (local)" to the client. Therefore, stream 0x1cannot be selected as a new stream identifier by a client that upgrades from HTTP/1.1.

The identifier of a newly established stream MUST be numerically greater than all streams that the initiatingendpoint has opened or reserved. This governs streams that are opened using a HEADERS frame and streamsthat are reserved using PUSH_PROMISE. An endpoint that receives an unexpected stream identifier MUSTrespond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

The first use of a new stream identifier implicitly closes all streams in the "idle" state that might have beeninitiated by that peer with a lower-valued stream identifier. For example, if a client sends a HEADERS frameon stream 7 without ever sending a frame on stream 5, then stream 5 transitions to the "closed" state when thefirst frame for stream 7 is sent or received.

Stream identifiers cannot be reused. Long-lived connections can result in an endpoint exhausting the availablerange of stream identifiers. A client that is unable to establish a new stream identifier can establish a newconnection for new streams. A server that is unable to establish a new stream identifier can send a GOAWAYframe so that the client is forced to open a new connection for new streams.

5.1.2. Stream Concurrency

A peer can limit the number of concurrently active streams using theSETTINGS_MAX_CONCURRENT_STREAMS parameter (see Section 6.5.2) within a SETTINGS frame.The maximum concurrent streams setting is specific to each endpoint and applies only to the peer that receivesthe setting. That is, clients specify the maximum number of concurrent streams the server can initiate, andservers specify the maximum number of concurrent streams the client can initiate.

Streams that are in the "open" state or in either of the "half-closed" states count toward the maximum numberof streams that an endpoint is permitted to open. Streams in any of these three states count toward the limitadvertised in the SETTINGS_MAX_CONCURRENT_STREAMS setting. Streams in either of the "reserved"states do not count toward the stream limit.

Endpoints MUST NOT exceed the limit set by their peer. An endpoint that receives a HEADERS frame thatcauses its advertised concurrent stream limit to be exceeded MUST treat this as a stream error (Section 5.4.2)of type PROTOCOL_ERROR or REFUSED_STREAM. The choice of error code determines whether theendpoint wishes to enable automatic retry (see Section 8.1.4) for details).

An endpoint that wishes to reduce the value of SETTINGS_MAX_CONCURRENT_STREAMS to a valuethat is below the current number of open streams can either close streams that exceed the new value or allowstreams to complete.

5.2. Flow Control

Using streams for multiplexing introduces contention over use of the TCP connection, resulting in blockedstreams. A flow-control scheme ensures that streams on the same connection do not destructively interfere witheach other. Flow control is used for both individual streams and for the connection as a whole.

HTTP/2 provides for flow control through use of the WINDOW_UPDATE frame (Section 6.9).

5.2.1. Flow-Control Principles

HTTP/2 stream flow control aims to allow a variety of flow-control algorithms to be used without requiringprotocol changes. Flow control in HTTP/2 has the following characteristics:



1. Flow control is specific to a connection. Both types of flow control are between the endpoints of a singlehop and not over the entire end-to-end path.

2. Flow control is based on WINDOW_UPDATE frames. Receivers advertise how many octets they areprepared to receive on a stream and for the entire connection. This is a credit-based scheme.

3. Flow control is directional with overall control provided by the receiver. A receiver MAY choose to setany window size that it desires for each stream and for the entire connection. A sender MUST respect flow-control limits imposed by a receiver. Clients, servers, and intermediaries all independently advertise theirflow-control window as a receiver and abide by the flow-control limits set by their peer when sending.

4. The initial value for the flow-control window is 65,535 octets for both new streams and the overallconnection.

5. The frame type determines whether flow control applies to a frame. Of the frames specified in thisdocument, only DATA frames are subject to flow control; all other frame types do not consume space inthe advertised flow-control window. This ensures that important control frames are not blocked by flowcontrol.

6. Flow control cannot be disabled.

7. HTTP/2 defines only the format and semantics of the WINDOW_UPDATE frame (Section 6.9). Thisdocument does not stipulate how a receiver decides when to send this frame or the value that it sends, nordoes it specify how a sender chooses to send packets. Implementations are able to select any algorithm thatsuits their needs.

Implementations are also responsible for managing how requests and responses are sent based on priority,choosing how to avoid head-of-line blocking for requests, and managing the creation of new streams.Algorithm choices for these could interact with any flow-control algorithm.

5.2.2. Appropriate Use of Flow Control

Flow control is defined to protect endpoints that are operating under resource constraints. For example, a proxyneeds to share memory between many connections and also might have a slow upstream connection and a fastdownstream one. Flow-control addresses cases where the receiver is unable to process data on one stream yetwants to continue to process other streams in the same connection.

Deployments that do not require this capability can advertise a flow-control window of the maximum size(231-1) and can maintain this window by sending a WINDOW_UPDATE frame when any data is received. Thiseffectively disables flow control for that receiver. Conversely, a sender is always subject to the flow-controlwindow advertised by the receiver.

Deployments with constrained resources (for example, memory) can employ flow control to limit the amountof memory a peer can consume. Note, however, that this can lead to suboptimal use of available networkresources if flow control is enabled without knowledge of the bandwidth-delay product (see [RFC7323]).

Even with full awareness of the current bandwidth-delay product, implementation of flow control can bedifficult. When using flow control, the receiver MUST read from the TCP receive buffer in a timely fashion.Failure to do so could lead to a deadlock when critical frames, such as WINDOW_UPDATE, are not read andacted upon.

5.3. Stream Priority

A client can assign a priority for a new stream by including prioritization information in the HEADERS frame(Section 6.2) that opens the stream. At any other time, the PRIORITY frame (Section 6.3) can be used tochange the priority of a stream.

The purpose of prioritization is to allow an endpoint to express how it would prefer its peer to allocateresources when managing concurrent streams. Most importantly, priority can be used to select streams fortransmitting frames when there is limited capacity for sending.



Streams can be prioritized by marking them as dependent on the completion of other streams (Section 5.3.1).Each dependency is assigned a relative weight, a number that is used to determine the relative proportion ofavailable resources that are assigned to streams dependent on the same stream.

Explicitly setting the priority for a stream is input to a prioritization process. It does not guarantee anyparticular processing or transmission order for the stream relative to any other stream. An endpoint cannot forcea peer to process concurrent streams in a particular order using priority. Expressing priority is therefore only asuggestion.

Prioritization information can be omitted from messages. Defaults are used prior to any explicit values beingprovided (Section 5.3.5).

5.3.1. Stream Dependencies

Each stream can be given an explicit dependency on another stream. Including a dependency expresses apreference to allocate resources to the identified stream rather than to the dependent stream.

A stream that is not dependent on any other stream is given a stream dependency of 0x0. In other words, thenon-existent stream 0 forms the root of the tree.

A stream that depends on another stream is a dependent stream. The stream upon which a stream is dependentis a parent stream. A dependency on a stream that is not currently in the tree — such as a stream in the "idle"state — results in that stream being given a default priority (Section 5.3.5).

When assigning a dependency on another stream, the stream is added as a new dependency of the parentstream. Dependent streams that share the same parent are not ordered with respect to each other. For example,if streams B and C are dependent on stream A, and if stream D is created with a dependency on stream A, thisresults in a dependency order of A followed by B, C, and D in any order.

A A / \ ==> /|\ B C B D C

Figure 3: Example of Default Dependency Creation

An exclusive flag allows for the insertion of a new level of dependencies. The exclusive flag causes the streamto become the sole dependency of its parent stream, causing other dependencies to become dependent on theexclusive stream. In the previous example, if stream D is created with an exclusive dependency on stream A,this results in D becoming the dependency parent of B and C.

A A | / \ ==> D B C / \ B C

Figure 4: Example of Exclusive Dependency Creation

Inside the dependency tree, a dependent stream SHOULD only be allocated resources if either all of thestreams that it depends on (the chain of parent streams up to 0x0) are closed or it is not possible to makeprogress on them.

A stream cannot depend on itself. An endpoint MUST treat this as a stream error (Section 5.4.2) of typePROTOCOL_ERROR.

5.3.2. Dependency Weighting

All dependent streams are allocated an integer weight between 1 and 256 (inclusive).



Streams with the same parent SHOULD be allocated resources proportionally based on their weight. Thus, ifstream B depends on stream A with weight 4, stream C depends on stream A with weight 12, and no progresscan be made on stream A, stream B ideally receives one-third of the resources allocated to stream C.

5.3.3. Reprioritization

Stream priorities are changed using the PRIORITY frame. Setting a dependency causes a stream to becomedependent on the identified parent stream.

Dependent streams move with their parent stream if the parent is reprioritized. Setting a dependency withthe exclusive flag for a reprioritized stream causes all the dependencies of the new parent stream to becomedependent on the reprioritized stream.

If a stream is made dependent on one of its own dependencies, the formerly dependent stream is first moved tobe dependent on the reprioritized stream's previous parent. The moved dependency retains its weight.

For example, consider an original dependency tree where B and C depend on A, D and E depend on C, and Fdepends on D. If A is made dependent on D, then D takes the place of A. All other dependency relationshipsstay the same, except for F, which becomes dependent on A if the reprioritization is exclusive.

x x x x | / \ | | A D A D D / \ / / \ / \ | B C ==> F B C ==> F A OR A / \ | / \ /|\ D E E B C B C F | | | F E E (intermediate) (non-exclusive) (exclusive)

Figure 5: Example of Dependency Reordering

5.3.4. Prioritization State Management

When a stream is removed from the dependency tree, its dependencies can be moved to become dependent onthe parent of the closed stream. The weights of new dependencies are recalculated by distributing the weight ofthe dependency of the closed stream proportionally based on the weights of its dependencies.

Streams that are removed from the dependency tree cause some prioritization information to be lost. Resourcesare shared between streams with the same parent stream, which means that if a stream in that set closes orbecomes blocked, any spare capacity allocated to a stream is distributed to the immediate neighbors of thestream. However, if the common dependency is removed from the tree, those streams share resources withstreams at the next highest level.

For example, assume streams A and B share a parent, and streams C and D both depend on stream A. Prior tothe removal of stream A, if streams A and D are unable to proceed, then stream C receives all the resourcesdedicated to stream A. If stream A is removed from the tree, the weight of stream A is divided between streamsC and D. If stream D is still unable to proceed, this results in stream C receiving a reduced proportion ofresources. For equal starting weights, C receives one third, rather than one half, of available resources.

It is possible for a stream to become closed while prioritization information that creates a dependency on thatstream is in transit. If a stream identified in a dependency has no associated priority information, then thedependent stream is instead assigned a default priority (Section 5.3.5). This potentially creates suboptimalprioritization, since the stream could be given a priority that is different from what is intended.

To avoid these problems, an endpoint SHOULD retain stream prioritization state for a period after streamsbecome closed. The longer state is retained, the lower the chance that streams are assigned incorrect or defaultpriority values.



Similarly, streams that are in the "idle" state can be assigned priority or become a parent of other streams. Thisallows for the creation of a grouping node in the dependency tree, which enables more flexible expressions ofpriority. Idle streams begin with a default priority (Section 5.3.5).

The retention of priority information for streams that are not counted toward the limit set bySETTINGS_MAX_CONCURRENT_STREAMS could create a large state burden for an endpoint. Therefore,the amount of prioritization state that is retained MAY be limited.

The amount of additional state an endpoint maintains for prioritization could be dependent on load;under high load, prioritization state can be discarded to limit resource commitments. In extremecases, an endpoint could even discard prioritization state for active or reserved streams. If a limit isapplied, endpoints SHOULD maintain state for at least as many streams as allowed by their setting forSETTINGS_MAX_CONCURRENT_STREAMS. Implementations SHOULD also attempt to retain state forstreams that are in active use in the priority tree.

If it has retained enough state to do so, an endpoint receiving a PRIORITY frame that changes the priority of aclosed stream SHOULD alter the dependencies of the streams that depend on it.

5.3.5. Default Priorities

All streams are initially assigned a non-exclusive dependency on stream 0x0. Pushed streams (Section 8.2)initially depend on their associated stream. In both cases, streams are assigned a default weight of 16.

5.4. Error Handling

HTTP/2 framing permits two classes of error:

• An error condition that renders the entire connection unusable is a connection error.

• An error in an individual stream is a stream error.

A list of error codes is included in Section 7.

5.4.1. Connection Error Handling

A connection error is any error that prevents further processing of the frame layer or corrupts any connectionstate.

An endpoint that encounters a connection error SHOULD first send a GOAWAY frame (Section 6.8) with thestream identifier of the last stream that it successfully received from its peer. The GOAWAY frame includesan error code that indicates why the connection is terminating. After sending the GOAWAY frame for an errorcondition, the endpoint MUST close the TCP connection.

It is possible that the GOAWAY will not be reliably received by the receiving endpoint ([RFC7230], Section6.6 describes how an immediate connection close can result in data loss). In the event of a connection error,GOAWAY only provides a best-effort attempt to communicate with the peer about why the connection is beingterminated.

An endpoint can end a connection at any time. In particular, an endpoint MAY choose to treat a stream erroras a connection error. Endpoints SHOULD send a GOAWAY frame when ending a connection, providing thatcircumstances permit it.

5.4.2. Stream Error Handling

A stream error is an error related to a specific stream that does not affect processing of other streams.

An endpoint that detects a stream error sends a RST_STREAM frame (Section 6.4) that contains the streamidentifier of the stream where the error occurred. The RST_STREAM frame includes an error code thatindicates the type of error.

A RST_STREAM is the last frame that an endpoint can send on a stream. The peer that sends theRST_STREAM frame MUST be prepared to receive any frames that were sent or enqueued for sending by



the remote peer. These frames can be ignored, except where they modify connection state (such as the statemaintained for header compression (Section 4.3) or flow control).

Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame for any stream. However, anendpoint MAY send additional RST_STREAM frames if it receives frames on a closed stream after more thana round-trip time. This behavior is permitted to deal with misbehaving implementations.

To avoid looping, an endpoint MUST NOT send a RST_STREAM in response to a RST_STREAM frame.

5.4.3. Connection Termination

If the TCP connection is closed or reset while streams remain in "open" or "half-closed" state, then the affectedstreams cannot be automatically retried (see Section 8.1.4 for details).

5.5. Extending HTTP/2

HTTP/2 permits extension of the protocol. Within the limitations described in this section, protocol extensionscan be used to provide additional services or alter any aspect of the protocol. Extensions are effective onlywithin the scope of a single HTTP/2 connection.

This applies to the protocol elements defined in this document. This does not affect the existing options forextending HTTP, such as defining new methods, status codes, or header fields.

Extensions are permitted to use new frame types (Section 4.1), new settings (Section 6.5.2), or new error codes(Section 7). Registries are established for managing these extension points: frame types (Section 11.2), settings(Section 11.3), and error codes (Section 11.4).

Implementations MUST ignore unknown or unsupported values in all extensible protocol elements.Implementations MUST discard frames that have unknown or unsupported types. This means that any ofthese extension points can be safely used by extensions without prior arrangement or negotiation. However,extension frames that appear in the middle of a header block (Section 4.3) are not permitted; these MUST betreated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

Extensions that could change the semantics of existing protocol components MUST be negotiated before beingused. For example, an extension that changes the layout of the HEADERS frame cannot be used until the peerhas given a positive signal that this is acceptable. In this case, it could also be necessary to coordinate when therevised layout comes into effect. Note that treating any frames other than DATA frames as flow controlled issuch a change in semantics and can only be done through negotiation.

This document doesn't mandate a specific method for negotiating the use of an extension but notes that a setting(Section 6.5.2) could be used for that purpose. If both peers set a value that indicates willingness to use theextension, then the extension can be used. If a setting is used for extension negotiation, the initial value MUSTbe defined in such a fashion that the extension is initially disabled.



6. Frame Definitions

This specification defines a number of frame types, each identified by a unique 8-bit type code. Each frametype serves a distinct purpose in the establishment and management either of the connection as a whole or ofindividual streams.

The transmission of specific frame types can alter the state of a connection. If endpoints fail to maintain asynchronized view of the connection state, successful communication within the connection will no longer bepossible. Therefore, it is important that endpoints have a shared comprehension of how the state is affected bythe use any given frame.

6.1. DATA

DATA frames (type=0x0) convey arbitrary, variable-length sequences of octets associated with a stream. Oneor more DATA frames are used, for instance, to carry HTTP request or response payloads.

DATA frames MAY also contain padding. Padding can be added to DATA frames to obscure the size ofmessages. Padding is a security feature; see Section 10.7.

+---------------+ |Pad Length? (8)| +---------------+-----------------------------------------------+ | Data (*) ... +---------------------------------------------------------------+ | Padding (*) ... +---------------------------------------------------------------+

Figure 6: DATA Frame Payload

The DATA frame contains the following fields:

Pad Length: An 8-bit field containing the length of the frame padding in units of octets. This field isconditional (as signified by a "?" in the diagram) and is only present if the PADDED flagis set.

Data: Application data. The amount of data is the remainder of the frame payload aftersubtracting the length of the other fields that are present.

Padding: Padding octets that contain no application semantic value. Padding octets MUST be set tozero when sending. A receiver is not obligated to verify padding but MAY treat non-zeropadding as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

The DATA frame defines the following flags:

END_STREAM (0x1): When set, bit 0 indicates that this frame is the last that the endpoint will sendfor the identified stream. Setting this flag causes the stream to enter one ofthe "half-closed" states or the "closed" state (Section 5.1).

PADDED (0x8): When set, bit 3 indicates that the Pad Length field and any padding that itdescribes are present.

DATA frames MUST be associated with a stream. If a DATA frame is received whose stream identifier field is0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

DATA frames are subject to flow control and can only be sent when a stream is in the "open" or "half-closed(remote)" state. The entire DATA frame payload is included in flow control, including the Pad Length andPadding fields if present. If a DATA frame is received whose stream is not in "open" or "half-closed (local)"state, the recipient MUST respond with a stream error (Section 5.4.2) of type STREAM_CLOSED.

The total number of padding octets is determined by the value of the Pad Length field. If the length of thepadding is the length of the frame payload or greater, the recipient MUST treat this as a connection error(Section 5.4.1) of type PROTOCOL_ERROR.



Note: A frame can be increased in size by one octet by including a Pad Length field with a value of zero.

6.2. HEADERS

The HEADERS frame (type=0x1) is used to open a stream (Section 5.1), and additionally carries a headerblock fragment. HEADERS frames can be sent on a stream in the "idle", "reserved (local)", "open", or "half-closed (remote)" state.

+---------------+ |Pad Length? (8)| +-+-------------+-----------------------------------------------+ |E| Stream Dependency? (31) | +-+-------------+-----------------------------------------------+ | Weight? (8) | +-+-------------+-----------------------------------------------+ | Header Block Fragment (*) ... +---------------------------------------------------------------+ | Padding (*) ... +---------------------------------------------------------------+

Figure 7: HEADERS Frame Payload

The HEADERS frame payload has the following fields:

Pad Length: An 8-bit field containing the length of the frame padding in units ofoctets. This field is only present if the PADDED flag is set.

E: A single-bit flag indicating that the stream dependency is exclusive(see Section 5.3). This field is only present if the PRIORITY flag isset.

Stream Dependency: A 31-bit stream identifier for the stream that this stream depends on(see Section 5.3). This field is only present if the PRIORITY flag isset.

Weight: An unsigned 8-bit integer representing a priority weight for thestream (see Section 5.3). Add one to the value to obtain a weightbetween 1 and 256. This field is only present if the PRIORITY flagis set.

Header Block Fragment: A header block fragment (Section 4.3).

Padding: Padding octets.

The HEADERS frame defines the following flags:

END_STREAM(0x1):When set, bit 0 indicates that the header block (Section 4.3) is the last that the endpoint will send for theidentified stream.

A HEADERS frame carries the END_STREAM flag that signals the end of a stream. However, a HEADERSframe with the END_STREAM flag set can be followed by CONTINUATION frames on the same stream.Logically, the CONTINUATION frames are part of the HEADERS frame.

END_HEADERS(0x4):When set, bit 2 indicates that this frame contains an entire header block (Section 4.3) and is not followed byany CONTINUATION frames.

A HEADERS frame without the END_HEADERS flag set MUST be followed by a CONTINUATION framefor the same stream. A receiver MUST treat the receipt of any other type of frame or a frame on a differentstream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

PADDED(0x8):When set, bit 3 indicates that the Pad Length field and any padding that it describes are present.



PRIORITY(0x20):When set, bit 5 indicates that the Exclusive Flag (E), Stream Dependency, and Weight fields are present; seeSection 5.3.

The payload of a HEADERS frame contains a header block fragment (Section 4.3). A header block that doesnot fit within a HEADERS frame is continued in a CONTINUATION frame (Section 6.10).

HEADERS frames MUST be associated with a stream. If a HEADERS frame is received whose streamidentifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of typePROTOCOL_ERROR.

The HEADERS frame changes the connection state as described in Section 4.3.

The HEADERS frame can include padding. Padding fields and flags are identical to those defined for DATAframes (Section 6.1). Padding that exceeds the size remaining for the header block fragment MUST be treatedas a PROTOCOL_ERROR.

Prioritization information in a HEADERS frame is logically equivalent to a separate PRIORITY frame,but inclusion in HEADERS avoids the potential for churn in stream prioritization when new streams arecreated. Prioritization fields in HEADERS frames subsequent to the first on a stream reprioritize the stream(Section 5.3.3).

6.3. PRIORITY

The PRIORITY frame (type=0x2) specifies the sender-advised priority of a stream (Section 5.3). It can be sentin any stream state, including idle or closed streams.

+-+-------------------------------------------------------------+ |E| Stream Dependency (31) | +-+-------------+-----------------------------------------------+ | Weight (8) | +-+-------------+

Figure 8: PRIORITY Frame Payload

The payload of a PRIORITY frame contains the following fields:

E: A single-bit flag indicating that the stream dependency is exclusive (seeSection 5.3).

Stream Dependency: A 31-bit stream identifier for the stream that this stream depends on (seeSection 5.3).

Weight: An unsigned 8-bit integer representing a priority weight for the stream (seeSection 5.3). Add one to the value to obtain a weight between 1 and 256.

The PRIORITY frame does not define any flags.

The PRIORITY frame always identifies a stream. If a PRIORITY frame is received with a stream identifier of0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

The PRIORITY frame can be sent on a stream in any state, though it cannot be sent between consecutiveframes that comprise a single header block (Section 4.3). Note that this frame could arrive after processing orframe sending has completed, which would cause it to have no effect on the identified stream. For a stream thatis in the "half-closed (remote)" or "closed" state, this frame can only affect processing of the identified streamand its dependent streams; it does not affect frame transmission on that stream.

The PRIORITY frame can be sent for a stream in the "idle" or "closed" state. This allows for thereprioritization of a group of dependent streams by altering the priority of an unused or closed parent stream.

A PRIORITY frame with a length other than 5 octets MUST be treated as a stream error (Section 5.4.2) of typeFRAME_SIZE_ERROR.



6.4. RST_STREAM

The RST_STREAM frame (type=0x3) allows for immediate termination of a stream. RST_STREAM is sent torequest cancellation of a stream or to indicate that an error condition has occurred.

+---------------------------------------------------------------+ | Error Code (32) | +---------------------------------------------------------------+

Figure 9: RST_STREAM Frame Payload

The RST_STREAM frame contains a single unsigned, 32-bit integer identifying the error code (Section 7). Theerror code indicates why the stream is being terminated.

The RST_STREAM frame does not define any flags.

The RST_STREAM frame fully terminates the referenced stream and causes it to enter the "closed" state. Afterreceiving a RST_STREAM on a stream, the receiver MUST NOT send additional frames for that stream, withthe exception of PRIORITY. However, after sending the RST_STREAM, the sending endpoint MUST beprepared to receive and process additional frames sent on the stream that might have been sent by the peer priorto the arrival of the RST_STREAM.

RST_STREAM frames MUST be associated with a stream. If a RST_STREAM frame is received witha stream identifier of 0x0, the recipient MUST treat this as a connection error (Section 5.4.1) of typePROTOCOL_ERROR.

RST_STREAM frames MUST NOT be sent for a stream in the "idle" state. If a RST_STREAM frameidentifying an idle stream is received, the recipient MUST treat this as a connection error (Section 5.4.1) oftype PROTOCOL_ERROR.

A RST_STREAM frame with a length other than 4 octets MUST be treated as a connection error(Section 5.4.1) of type FRAME_SIZE_ERROR.

6.5. SETTINGS

The SETTINGS frame (type=0x4) conveys configuration parameters that affect how endpoints communicate,such as preferences and constraints on peer behavior. The SETTINGS frame is also used to acknowledge thereceipt of those parameters. Individually, a SETTINGS parameter can also be referred to as a "setting".

SETTINGS parameters are not negotiated; they describe characteristics of the sending peer, which are usedby the receiving peer. Different values for the same parameter can be advertised by each peer. For example,a client might set a high initial flow-control window, whereas a server might set a lower value to conserveresources.

A SETTINGS frame MUST be sent by both endpoints at the start of a connection and MAY be sent at anyother time by either endpoint over the lifetime of the connection. Implementations MUST support all of theparameters defined by this specification.

Each parameter in a SETTINGS frame replaces any existing value for that parameter. Parameters are processedin the order in which they appear, and a receiver of a SETTINGS frame does not need to maintain any stateother than the current value of its parameters. Therefore, the value of a SETTINGS parameter is the last valuethat is seen by a receiver.

SETTINGS parameters are acknowledged by the receiving peer. To enable this, the SETTINGS frame definesthe following flag:

ACK (0x1): When set, bit 0 indicates that this frame acknowledges receipt and application of thepeer's SETTINGS frame. When this bit is set, the payload of the SETTINGS frameMUST be empty. Receipt of a SETTINGS frame with the ACK flag set and a lengthfield value other than 0 MUST be treated as a connection error (Section 5.4.1) of



type FRAME_SIZE_ERROR. For more information, see Section 6.5.3 ("SettingsSynchronization").

SETTINGS frames always apply to a connection, never a single stream. The stream identifier for a SETTINGSframe MUST be zero (0x0). If an endpoint receives a SETTINGS frame whose stream identifier fieldis anything other than 0x0, the endpoint MUST respond with a connection error (Section 5.4.1) of typePROTOCOL_ERROR.

The SETTINGS frame affects connection state. A badly formed or incomplete SETTINGS frame MUST betreated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

A SETTINGS frame with a length other than a multiple of 6 octets MUST be treated as a connection error(Section 5.4.1) of type FRAME_SIZE_ERROR.

6.5.1. SETTINGS Format

The payload of a SETTINGS frame consists of zero or more parameters, each consisting of an unsigned 16-bitsetting identifier and an unsigned 32-bit value.

+-------------------------------+ | Identifier (16) | +-------------------------------+-------------------------------+ | Value (32) | +---------------------------------------------------------------+

Figure 10: Setting Format

6.5.2. Defined SETTINGS Parameters

The following parameters are defined:

SETTINGS_HEADER_TABLE_SIZE(0x1):Allows the sender to inform the remote endpoint of the maximum size of the header compression table usedto decode header blocks, in octets. The encoder can select any size equal to or less than this value by usingsignaling specific to the header compression format inside a header block (see [COMPRESSION]). The initialvalue is 4,096 octets.

SETTINGS_ENABLE_PUSH(0x2):This setting can be used to disable server push (Section 8.2). An endpoint MUST NOT send aPUSH_PROMISE frame if it receives this parameter set to a value of 0. An endpoint that has both set thisparameter to 0 and had it acknowledged MUST treat the receipt of a PUSH_PROMISE frame as a connectionerror (Section 5.4.1) of type PROTOCOL_ERROR.

The initial value is 1, which indicates that server push is permitted. Any value other than 0 or 1 MUST betreated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

SETTINGS_MAX_CONCURRENT_STREAMS(0x3):Indicates the maximum number of concurrent streams that the sender will allow. This limit is directional: itapplies to the number of streams that the sender permits the receiver to create. Initially, there is no limit to thisvalue. It is recommended that this value be no smaller than 100, so as to not unnecessarily limit parallelism.

A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be treated as special byendpoints. A zero value does prevent the creation of new streams; however, this can also happen for any limitthat is exhausted with active streams. Servers SHOULD only set a zero value for short durations; if a serverdoes not wish to accept requests, closing the connection is more appropriate.

SETTINGS_INITIAL_WINDOW_SIZE(0x4):Indicates the sender's initial window size (in octets) for stream-level flow control. The initial value is 216-1(65,535) octets.

This setting affects the window size of all streams (see Section 6.9.2).

Values above the maximum flow-control window size of 231-1 MUST be treated as a connection error(Section 5.4.1) of type FLOW_CONTROL_ERROR.

SETTINGS_MAX_FRAME_SIZE(0x5):Indicates the size of the largest frame payload that the sender is willing to receive, in octets.



The initial value is 214 (16,384) octets. The value advertised by an endpoint MUST be between this initial valueand the maximum allowed frame size (224-1 or 16,777,215 octets), inclusive. Values outside this range MUSTbe treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

SETTINGS_MAX_HEADER_LIST_SIZE(0x6):This advisory setting informs a peer of the maximum size of header list that the sender is prepared to accept, inoctets. The value is based on the uncompressed size of header fields, including the length of the name and valuein octets plus an overhead of 32 octets for each header field.

For any given request, a lower limit than what is advertised MAY be enforced. The initial value of this settingis unlimited.

An endpoint that receives a SETTINGS frame with any unknown or unsupported identifier MUST ignore thatsetting.

6.5.3. Settings Synchronization

Most values in SETTINGS benefit from or require an understanding of when the peer has received andapplied the changed parameter values. In order to provide such synchronization timepoints, the recipient of aSETTINGS frame in which the ACK flag is not set MUST apply the updated parameters as soon as possibleupon receipt.

The values in the SETTINGS frame MUST be processed in the order they appear, with no other frameprocessing between values. Unsupported parameters MUST be ignored. Once all values have been processed,the recipient MUST immediately emit a SETTINGS frame with the ACK flag set. Upon receiving aSETTINGS frame with the ACK flag set, the sender of the altered parameters can rely on the setting havingbeen applied.

If the sender of a SETTINGS frame does not receive an acknowledgement within a reasonable amount of time,it MAY issue a connection error (Section 5.4.1) of type SETTINGS_TIMEOUT.

6.6. PUSH_PROMISE

The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint in advance of streams the senderintends to initiate. The PUSH_PROMISE frame includes the unsigned 31-bit identifier of the stream theendpoint plans to create along with a set of headers that provide additional context for the stream. Section 8.2contains a thorough description of the use of PUSH_PROMISE frames.

+---------------+ |Pad Length? (8)| +-+-------------+-----------------------------------------------+ |R| Promised Stream ID (31) | +-+-----------------------------+-------------------------------+ | Header Block Fragment (*) ... +---------------------------------------------------------------+ | Padding (*) ... +---------------------------------------------------------------+

Figure 11: PUSH_PROMISE Payload Format

The PUSH_PROMISE frame payload has the following fields:

Pad Length: An 8-bit field containing the length of the frame padding in units ofoctets. This field is only present if the PADDED flag is set.

R: A single reserved bit.

Promised Stream ID: An unsigned 31-bit integer that identifies the stream that is reservedby the PUSH_PROMISE. The promised stream identifier MUSTbe a valid choice for the next stream sent by the sender (see "newstream identifier" in Section 5.1.1).



Header Block Fragment: A header block fragment (Section 4.3) containing request headerfields.

Padding: Padding octets.

The PUSH_PROMISE frame defines the following flags:

END_HEADERS(0x4):When set, bit 2 indicates that this frame contains an entire header block (Section 4.3) and is not followed byany CONTINUATION frames.

A PUSH_PROMISE frame without the END_HEADERS flag set MUST be followed by a CONTINUATIONframe for the same stream. A receiver MUST treat the receipt of any other type of frame or a frame on adifferent stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

PADDED(0x8):When set, bit 3 indicates that the Pad Length field and any padding that it describes are present.

PUSH_PROMISE frames MUST only be sent on a peer-initiated stream that is in either the "open" or "half-closed (remote)" state. The stream identifier of a PUSH_PROMISE frame indicates the stream it is associatedwith. If the stream identifier field specifies the value 0x0, a recipient MUST respond with a connection error(Section 5.4.1) of type PROTOCOL_ERROR.

Promised streams are not required to be used in the order they are promised. The PUSH_PROMISE onlyreserves stream identifiers for later use.

PUSH_PROMISE MUST NOT be sent if the SETTINGS_ENABLE_PUSH setting of the peer endpoint isset to 0. An endpoint that has set this setting and has received acknowledgement MUST treat the receipt of aPUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

Recipients of PUSH_PROMISE frames can choose to reject promised streams by returning a RST_STREAMreferencing the promised stream identifier back to the sender of the PUSH_PROMISE.

A PUSH_PROMISE frame modifies the connection state in two ways. First, the inclusion of a header block(Section 4.3) potentially modifies the state maintained for header compression. Second, PUSH_PROMISE alsoreserves a stream for later use, causing the promised stream to enter the "reserved" state. A sender MUST NOTsend a PUSH_PROMISE on a stream unless that stream is either "open" or "half-closed (remote)"; the senderMUST ensure that the promised stream is a valid choice for a new stream identifier (Section 5.1.1) (that is, thepromised stream MUST be in the "idle" state).

Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame causes the stream state tobecome indeterminate. A receiver MUST treat the receipt of a PUSH_PROMISE on a stream that is neither"open" nor "half-closed (local)" as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. However,an endpoint that has sent RST_STREAM on the associated stream MUST handle PUSH_PROMISE framesthat might have been created before the RST_STREAM frame is received and processed.

A receiver MUST treat the receipt of a PUSH_PROMISE that promises an illegal stream identifier(Section 5.1.1) as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Note that an illegal streamidentifier is an identifier for a stream that is not currently in the "idle" state.

The PUSH_PROMISE frame can include padding. Padding fields and flags are identical to those defined forDATA frames (Section 6.1).

6.7. PING

The PING frame (type=0x6) is a mechanism for measuring a minimal round-trip time from the sender, as wellas determining whether an idle connection is still functional. PING frames can be sent from any endpoint.



+---------------------------------------------------------------+ | | | Opaque Data (64) | | | +---------------------------------------------------------------+

Figure 12: PING Payload Format

In addition to the frame header, PING frames MUST contain 8 octets of opaque data in the payload. A sendercan include any value it chooses and use those octets in any fashion.

Receivers of a PING frame that does not include an ACK flag MUST send a PING frame with the ACK flagset in response, with an identical payload. PING responses SHOULD be given higher priority than any otherframe.

The PING frame defines the following flags:

ACK (0x1): When set, bit 0 indicates that this PING frame is a PING response. An endpoint MUST setthis flag in PING responses. An endpoint MUST NOT respond to PING frames containingthis flag.

PING frames are not associated with any individual stream. If a PING frame is received with a stream identifierfield value other than 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of typePROTOCOL_ERROR.

Receipt of a PING frame with a length field value other than 8 MUST be treated as a connection error(Section 5.4.1) of type FRAME_SIZE_ERROR.

6.8. GOAWAY

The GOAWAY frame (type=0x7) is used to initiate shutdown of a connection or to signal serious errorconditions. GOAWAY allows an endpoint to gracefully stop accepting new streams while still finishingprocessing of previously established streams. This enables administrative actions, like server maintenance.

There is an inherent race condition between an endpoint starting new streams and the remote sending aGOAWAY frame. To deal with this case, the GOAWAY contains the stream identifier of the last peer-initiatedstream that was or might be processed on the sending endpoint in this connection. For instance, if the serversends a GOAWAY frame, the identified stream is the highest-numbered stream initiated by the client.

Once sent, the sender will ignore frames sent on streams initiated by the receiver if the stream has an identifierhigher than the included last stream identifier. Receivers of a GOAWAY frame MUST NOT open additionalstreams on the connection, although a new connection can be established for new streams.

If the receiver of the GOAWAY has sent data on streams with a higher stream identifier than what is indicatedin the GOAWAY frame, those streams are not or will not be processed. The receiver of the GOAWAY framecan treat the streams as though they had never been created at all, thereby allowing those streams to be retriedlater on a new connection.

Endpoints SHOULD always send a GOAWAY frame before closing a connection so that the remote peer canknow whether a stream has been partially processed or not. For example, if an HTTP client sends a POST at thesame time that a server closes a connection, the client cannot know if the server started to process that POSTrequest if the server does not send a GOAWAY frame to indicate what streams it might have acted on.

An endpoint might choose to close a connection without sending a GOAWAY for misbehaving peers.

A GOAWAY frame might not immediately precede closing of the connection; a receiver of a GOAWAY thathas no more use for the connection SHOULD still send a GOAWAY frame before terminating the connection.



+-+-------------------------------------------------------------+ |R| Last-Stream-ID (31) | +-+-------------------------------------------------------------+ | Error Code (32) | +---------------------------------------------------------------+ | Additional Debug Data (*) | +---------------------------------------------------------------+

Figure 13: GOAWAY Payload Format

The GOAWAY frame does not define any flags.

The GOAWAY frame applies to the connection, not a specific stream. An endpoint MUST treat aGOAWAY frame with a stream identifier other than 0x0 as a connection error (Section 5.4.1) of typePROTOCOL_ERROR.

The last stream identifier in the GOAWAY frame contains the highest-numbered stream identifier for whichthe sender of the GOAWAY frame might have taken some action on or might yet take action on. All streams upto and including the identified stream might have been processed in some way. The last stream identifier can beset to 0 if no streams were processed.

Note: In this context, "processed" means that some data from the stream was passed to some higher layerof software that might have taken some action as a result.

If a connection terminates without a GOAWAY frame, the last stream identifier is effectively the highestpossible stream identifier.

On streams with lower- or equal-numbered identifiers that were not closed completely prior to the connectionbeing closed, reattempting requests, transactions, or any protocol activity is not possible, with the exceptionof idempotent actions like HTTP GET, PUT, or DELETE. Any protocol activity that uses higher-numberedstreams can be safely retried using a new connection.

Activity on streams numbered lower or equal to the last stream identifier might still complete successfully.The sender of a GOAWAY frame might gracefully shut down a connection by sending a GOAWAY frame,maintaining the connection in an "open" state until all in-progress streams complete.

An endpoint MAY send multiple GOAWAY frames if circumstances change. For instance, an endpoint thatsends GOAWAY with NO_ERROR during graceful shutdown could subsequently encounter a condition thatrequires immediate termination of the connection. The last stream identifier from the last GOAWAY framereceived indicates which streams could have been acted upon. Endpoints MUST NOT increase the value theysend in the last stream identifier, since the peers might already have retried unprocessed requests on anotherconnection.

A client that is unable to retry requests loses all requests that are in flight when the server closes the connection.This is especially true for intermediaries that might not be serving clients using HTTP/2. A server that isattempting to gracefully shut down a connection SHOULD send an initial GOAWAY frame with the laststream identifier set to 231-1 and a NO_ERROR code. This signals to the client that a shutdown is imminentand that initiating further requests is prohibited. After allowing time for any in-flight stream creation (at leastone round-trip time), the server can send another GOAWAY frame with an updated last stream identifier. Thisensures that a connection can be cleanly shut down without losing requests.

After sending a GOAWAY frame, the sender can discard frames for streams initiated by the receiver withidentifiers higher than the identified last stream. However, any frames that alter connection state cannot becompletely ignored. For instance, HEADERS, PUSH_PROMISE, and CONTINUATION frames MUST beminimally processed to ensure the state maintained for header compression is consistent (see Section 4.3);similarly, DATA frames MUST be counted toward the connection flow-control window. Failure to processthese frames can cause flow control or header compression state to become unsynchronized.

The GOAWAY frame also contains a 32-bit error code (Section 7) that contains the reason for closing theconnection.



Endpoints MAY append opaque data to the payload of any GOAWAY frame. Additional debug data isintended for diagnostic purposes only and carries no semantic value. Debug information could contain security-or privacy-sensitive data. Logged or otherwise persistently stored debug data MUST have adequate safeguardsto prevent unauthorized access.

6.9. WINDOW_UPDATE

The WINDOW_UPDATE frame (type=0x8) is used to implement flow control; see Section 5.2 for anoverview.

Flow control operates at two levels: on each individual stream and on the entire connection.

Both types of flow control are hop by hop, that is, only between the two endpoints. Intermediaries do notforward WINDOW_UPDATE frames between dependent connections. However, throttling of data transfer byany receiver can indirectly cause the propagation of flow-control information toward the original sender.

Flow control only applies to frames that are identified as being subject to flow control. Of the frame typesdefined in this document, this includes only DATA frames. Frames that are exempt from flow controlMUST be accepted and processed, unless the receiver is unable to assign resources to handling the frame.A receiver MAY respond with a stream error (Section 5.4.2) or connection error (Section 5.4.1) of typeFLOW_CONTROL_ERROR if it is unable to accept a frame.

+-+-------------------------------------------------------------+ |R| Window Size Increment (31) | +-+-------------------------------------------------------------+

Figure 14: WINDOW_UPDATE Payload Format

The payload of a WINDOW_UPDATE frame is one reserved bit plus an unsigned 31-bit integer indicating thenumber of octets that the sender can transmit in addition to the existing flow-control window. The legal rangefor the increment to the flow-control window is 1 to 231-1 (2,147,483,647) octets.

The WINDOW_UPDATE frame does not define any flags.

The WINDOW_UPDATE frame can be specific to a stream or to the entire connection. In the former case,the frame's stream identifier indicates the affected stream; in the latter, the value "0" indicates that the entireconnection is the subject of the frame.

A receiver MUST treat the receipt of a WINDOW_UPDATE frame with an flow-control window incrementof 0 as a stream error (Section 5.4.2) of type PROTOCOL_ERROR; errors on the connection flow-controlwindow MUST be treated as a connection error (Section 5.4.1).

WINDOW_UPDATE can be sent by a peer that has sent a frame bearing the END_STREAM flag. This meansthat a receiver could receive a WINDOW_UPDATE frame on a "half-closed (remote)" or "closed" stream. Areceiver MUST NOT treat this as an error (see Section 5.1).

A receiver that receives a flow-controlled frame MUST always account for its contribution against theconnection flow-control window, unless the receiver treats this as a connection error (Section 5.4.1). This isnecessary even if the frame is in error. The sender counts the frame toward the flow-control window, but if thereceiver does not, the flow-control window at the sender and receiver can become different.

A WINDOW_UPDATE frame with a length other than 4 octets MUST be treated as a connection error(Section 5.4.1) of type FRAME_SIZE_ERROR.

6.9.1. The Flow-Control Window

Flow control in HTTP/2 is implemented using a window kept by each sender on every stream. The flow-controlwindow is a simple integer value that indicates how many octets of data the sender is permitted to transmit; assuch, its size is a measure of the buffering capacity of the receiver.



Two flow-control windows are applicable: the stream flow-control window and the connection flow-control window. The sender MUST NOT send a flow-controlled frame with a length that exceeds the spaceavailable in either of the flow-control windows advertised by the receiver. Frames with zero length with theEND_STREAM flag set (that is, an empty DATA frame) MAY be sent if there is no available space in eitherflow-control window.

For flow-control calculations, the 9-octet frame header is not counted.

After sending a flow-controlled frame, the sender reduces the space available in both windows by the length ofthe transmitted frame.

The receiver of a frame sends a WINDOW_UPDATE frame as it consumes data and frees up space in flow-control windows. Separate WINDOW_UPDATE frames are sent for the stream- and connection-level flow-control windows.

A sender that receives a WINDOW_UPDATE frame updates the corresponding window by the amountspecified in the frame.

A sender MUST NOT allow a flow-control window to exceed 231-1 octets. If a sender receives aWINDOW_UPDATE that causes a flow-control window to exceed this maximum, it MUST terminateeither the stream or the connection, as appropriate. For streams, the sender sends a RST_STREAM with anerror code of FLOW_CONTROL_ERROR; for the connection, a GOAWAY frame with an error code ofFLOW_CONTROL_ERROR is sent.

Flow-controlled frames from the sender and WINDOW_UPDATE frames from the receiver are completelyasynchronous with respect to each other. This property allows a receiver to aggressively update the windowsize kept by the sender to prevent streams from stalling.

6.9.2. Initial Flow-Control Window Size

When an HTTP/2 connection is first established, new streams are created with an initial flow-control windowsize of 65,535 octets. The connection flow-control window is also 65,535 octets. Both endpoints can adjust theinitial window size for new streams by including a value for SETTINGS_INITIAL_WINDOW_SIZE in theSETTINGS frame that forms part of the connection preface. The connection flow-control window can only bechanged using WINDOW_UPDATE frames.

Prior to receiving a SETTINGS frame that sets a value for SETTINGS_INITIAL_WINDOW_SIZE, anendpoint can only use the default initial window size when sending flow-controlled frames. Similarly, theconnection flow-control window is set to the default initial window size until a WINDOW_UPDATE frame isreceived.

In addition to changing the flow-control window for streams that are not yet active, a SETTINGS frame canalter the initial flow-control window size for streams with active flow-control windows (that is, streams in the"open" or "half-closed (remote)" state). When the value of SETTINGS_INITIAL_WINDOW_SIZE changes, areceiver MUST adjust the size of all stream flow-control windows that it maintains by the difference betweenthe new value and the old value.

A change to SETTINGS_INITIAL_WINDOW_SIZE can cause the available space in a flow-control windowto become negative. A sender MUST track the negative flow-control window and MUST NOT send new flow-controlled frames until it receives WINDOW_UPDATE frames that cause the flow-control window to becomepositive.

For example, if the client sends 60 KB immediately on connection establishment and the server sets the initialwindow size to be 16 KB, the client will recalculate the available flow-control window to be -44 KB on receiptof the SETTINGS frame. The client retains a negative flow-control window until WINDOW_UPDATE framesrestore the window to being positive, after which the client can resume sending.

A SETTINGS frame cannot alter the connection flow-control window.



An endpoint MUST treat a change to SETTINGS_INITIAL_WINDOW_SIZE that causes anyflow-control window to exceed the maximum size as a connection error (Section 5.4.1) of typeFLOW_CONTROL_ERROR.

6.9.3. Reducing the Stream Window Size

A receiver that wishes to use a smaller flow-control window than the current size can send a new SETTINGSframe. However, the receiver MUST be prepared to receive data that exceeds this window size, since the sendermight send data that exceeds the lower limit prior to processing the SETTINGS frame.

After sending a SETTINGS frame that reduces the initial flow-control window size, a receiver MAY continueto process streams that exceed flow-control limits. Allowing streams to continue does not allow the receiverto immediately reduce the space it reserves for flow-control windows. Progress on these streams can also stall,since WINDOW_UPDATE frames are needed to allow the sender to resume sending. The receiver MAYinstead send a RST_STREAM with an error code of FLOW_CONTROL_ERROR for the affected streams.

6.10. CONTINUATION

The CONTINUATION frame (type=0x9) is used to continue a sequence of header block fragments(Section 4.3). Any number of CONTINUATION frames can be sent, as long as the preceding frame ison the same stream and is a HEADERS, PUSH_PROMISE, or CONTINUATION frame without theEND_HEADERS flag set.

+---------------------------------------------------------------+ | Header Block Fragment (*) ... +---------------------------------------------------------------+

Figure 15: CONTINUATION Frame Payload

The CONTINUATION frame payload contains a header block fragment (Section 4.3).

The CONTINUATION frame defines the following flag:

END_HEADERS(0x4):When set, bit 2 indicates that this frame ends a header block (Section 4.3).

If the END_HEADERS bit is not set, this frame MUST be followed by another CONTINUATION frame. Areceiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connectionerror (Section 5.4.1) of type PROTOCOL_ERROR.

The CONTINUATION frame changes the connection state as defined in Section 4.3.

CONTINUATION frames MUST be associated with a stream. If a CONTINUATION frame is received whosestream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of typePROTOCOL_ERROR.

A CONTINUATION frame MUST be preceded by a HEADERS, PUSH_PROMISE or CONTINUATIONframe without the END_HEADERS flag set. A recipient that observes violation of this rule MUST respondwith a connection error (Section 5.4.1) of type PROTOCOL_ERROR.



7. Error Codes

Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY frames to convey the reasons forthe stream or connection error.

Error codes share a common code space. Some error codes apply only to either streams or the entire connectionand have no defined semantics in the other context.

The following error codes are defined:

NO_ERROR (0x0): The associated condition is not a result of an error. Forexample, a GOAWAY might include this code to indicategraceful shutdown of a connection.

PROTOCOL_ERROR (0x1): The endpoint detected an unspecific protocol error. Thiserror is for use when a more specific error code is notavailable.

INTERNAL_ERROR (0x2): The endpoint encountered an unexpected internal error.

FLOW_CONTROL_ERROR (0x3): The endpoint detected that its peer violated the flow-controlprotocol.

SETTINGS_TIMEOUT (0x4): The endpoint sent a SETTINGS frame but did not receivea response in a timely manner. See Section 6.5.3 ("SettingsSynchronization").

STREAM_CLOSED (0x5): The endpoint received a frame after a stream was half-closed.

FRAME_SIZE_ERROR (0x6): The endpoint received a frame with an invalid size.

REFUSED_STREAM (0x7): The endpoint refused the stream prior to performing anyapplication processing (see Section 8.1.4 for details).

CANCEL (0x8): Used by the endpoint to indicate that the stream is nolonger needed.

COMPRESSION_ERROR (0x9): The endpoint is unable to maintain the header compressioncontext for the connection.

CONNECT_ERROR (0xa): The connection established in response to a CONNECTrequest (Section 8.3) was reset or abnormally closed.

ENHANCE_YOUR_CALM (0xb): The endpoint detected that its peer is exhibiting a behaviorthat might be generating excessive load.

INADEQUATE_SECURITY (0xc): The underlying transport has properties that do not meetminimum security requirements (see Section 9.2).

HTTP_1_1_REQUIRED (0xd): The endpoint requires that HTTP/1.1 be used instead ofHTTP/2.

Unknown or unsupported error codes MUST NOT trigger any special behavior. These MAY be treated by animplementation as being equivalent to INTERNAL_ERROR.



8. HTTP Message Exchanges

HTTP/2 is intended to be as compatible as possible with current uses of HTTP. This means that, from theapplication perspective, the features of the protocol are largely unchanged. To achieve this, all request andresponse semantics are preserved, although the syntax of conveying those semantics has changed.

Thus, the specification and requirements of HTTP/1.1 Semantics and Content [RFC7231], ConditionalRequests [RFC7232], Range Requests [RFC7233], Caching [RFC7234], and Authentication [RFC7235] areapplicable to HTTP/2. Selected portions of HTTP/1.1 Message Syntax and Routing [RFC7230], such as theHTTP and HTTPS URI schemes, are also applicable in HTTP/2, but the expression of those semantics for thisprotocol are defined in the sections below.

8.1. HTTP Request/Response Exchange

A client sends an HTTP request on a new stream, using a previously unused stream identifier (Section 5.1.1). Aserver sends an HTTP response on the same stream as the request.

An HTTP message (request or response) consists of:

1. for a response only, zero or more HEADERS frames (each followed by zero or more CONTINUATIONframes) containing the message headers of informational (1xx) HTTP responses (see [RFC7230], Section3.2 and [RFC7231], Section 6.2),

2. one HEADERS frame (followed by zero or more CONTINUATION frames) containing the messageheaders (see [RFC7230], Section 3.2),

3. zero or more DATA frames containing the payload body (see [RFC7230], Section 3.3), and

4. optionally, one HEADERS frame, followed by zero or more CONTINUATION frames containing thetrailer-part, if present (see [RFC7230], Section 4.1.2).

The last frame in the sequence bears an END_STREAM flag, noting that a HEADERS frame bearing theEND_STREAM flag can be followed by CONTINUATION frames that carry any remaining portions of theheader block.

Other frames (from any stream) MUST NOT occur between the HEADERS frame and any CONTINUATIONframes that might follow.

HTTP/2 uses DATA frames to carry message payloads. The chunked transfer encoding defined in Section 4.1of [RFC7230] MUST NOT be used in HTTP/2.

Trailing header fields are carried in a header block that also terminates the stream. Such a header block is asequence starting with a HEADERS frame, followed by zero or more CONTINUATION frames, where theHEADERS frame bears an END_STREAM flag. Header blocks after the first that do not terminate the streamare not part of an HTTP request or response.

A HEADERS frame (and associated CONTINUATION frames) can only appear at the start or end of a stream.An endpoint that receives a HEADERS frame without the END_STREAM flag set after receiving a final (non-informational) status code MUST treat the corresponding request or response as malformed (Section 8.1.2.6).

An HTTP request/response exchange fully consumes a single stream. A request starts with the HEADERSframe that puts the stream into an "open" state. The request ends with a frame bearing END_STREAM, whichcauses the stream to become "half-closed (local)" for the client and "half-closed (remote)" for the server. Aresponse starts with a HEADERS frame and ends with a frame bearing END_STREAM, which places thestream in the "closed" state.

An HTTP response is complete after the server sends — or the client receives — a frame with theEND_STREAM flag set (including any CONTINUATION frames needed to complete a header block). Aserver can send a complete response prior to the client sending an entire request if the response does not dependon any portion of the request that has not been sent and received. When this is true, a server MAY requestthat the client abort transmission of a request without error by sending a RST_STREAM with an error code ofNO_ERROR after sending a complete response (i.e., a frame with the END_STREAM flag). Clients MUST



NOT discard responses as a result of receiving such a RST_STREAM, though clients can always discardresponses at their discretion for other reasons.

8.1.1. Upgrading from HTTP/2

HTTP/2 removes support for the 101 (Switching Protocols) informational status code ([RFC7231], Section6.2.2).

The semantics of 101 (Switching Protocols) aren't applicable to a multiplexed protocol. Alternative protocolsare able to use the same mechanisms that HTTP/2 uses to negotiate their use (see Section 3).

8.1.2. HTTP Header Fields

HTTP header fields carry information as a series of key-value pairs. For a listing of registered HTTP headers,see the "Message Header Field" registry maintained at <https://www.iana.org/assignments/message-headers>.

Just as in HTTP/1.x, header field names are strings of ASCII characters that are compared in a case-insensitivefashion. However, header field names MUST be converted to lowercase prior to their encoding in HTTP/2. Arequest or response containing uppercase header field names MUST be treated as malformed (Section 8.1.2.6).

8.1.2.1. Pseudo-Header Fields

While HTTP/1.x used the message start-line (see [RFC7230], Section 3.1) to convey the target URI, themethod of the request, and the status code for the response, HTTP/2 uses special pseudo-header fieldsbeginning with ':' character (ASCII 0x3a) for this purpose.

Pseudo-header fields are not HTTP header fields. Endpoints MUST NOT generate pseudo-header fields otherthan those defined in this document.

Pseudo-header fields are only valid in the context in which they are defined. Pseudo-header fields defined forrequests MUST NOT appear in responses; pseudo-header fields defined for responses MUST NOT appear inrequests. Pseudo-header fields MUST NOT appear in trailers. Endpoints MUST treat a request or response thatcontains undefined or invalid pseudo-header fields as malformed (Section 8.1.2.6).

All pseudo-header fields MUST appear in the header block before regular header fields. Any request orresponse that contains a pseudo-header field that appears in a header block after a regular header field MUSTbe treated as malformed (Section 8.1.2.6).

8.1.2.2. Connection-Specific Header Fields

HTTP/2 does not use the Connection header field to indicate connection-specific header fields; in thisprotocol, connection-specific metadata is conveyed by other means. An endpoint MUST NOT generate anHTTP/2 message containing connection-specific header fields; any message containing connection-specificheader fields MUST be treated as malformed (Section 8.1.2.6).

The only exception to this is the TE header field, which MAY be present in an HTTP/2 request; when it is, itMUST NOT contain any value other than "trailers".

This means that an intermediary transforming an HTTP/1.x message to HTTP/2 will need to remove anyheader fields nominated by the Connection header field, along with the Connection header field itself. Suchintermediaries SHOULD also remove other connection-specific header fields, such as Keep-Alive, Proxy-Connection, Transfer-Encoding, and Upgrade, even if they are not nominated by the Connection header field.

Note: HTTP/2 purposefully does not support upgrade to another protocol. The handshake methodsdescribed in Section 3 are believed sufficient to negotiate the use of alternative protocols.

8.1.2.3. Request Pseudo-Header Fields

The following pseudo-header fields are defined for HTTP/2 requests:

• The :method pseudo-header field includes the HTTP method ([RFC7231], Section 4).


https://www.iana.org/assignments/message-headers


• The :scheme pseudo-header field includes the scheme portion of the target URI ([RFC3986], Section3.1).

:scheme is not restricted to http and https schemed URIs. A proxy or gateway can translate requestsfor non-HTTP schemes, enabling the use of HTTP to interact with non-HTTP services.

• The :authority pseudo-header field includes the authority portion of the target URI ([RFC3986],Section 3.2). The authority MUST NOT include the deprecated userinfo subcomponent for http orhttps schemed URIs.

To ensure that the HTTP/1.1 request line can be reproduced accurately, this pseudo-header field MUST beomitted when translating from an HTTP/1.1 request that has a request target in origin or asterisk form (see[RFC7230], Section 5.3). Clients that generate HTTP/2 requests directly SHOULD use the :authoritypseudo-header field instead of the Host header field. An intermediary that converts an HTTP/2 request toHTTP/1.1 MUST create a Host header field if one is not present in a request by copying the value of the:authority pseudo-header field.

• The :path pseudo-header field includes the path and query parts of the target URI (the path-absolute production and optionally a '?' character followed by the query production (see Sections 3.3and 3.4 of [RFC3986]). A request in asterisk form includes the value '*' for the :path pseudo-header field.

This pseudo-header field MUST NOT be empty for http or https URIs; http or https URIs thatdo not contain a path component MUST include a value of '/'. The exception to this rule is an OPTIONSrequest for an http or https URI that does not include a path component; these MUST include a :pathpseudo-header field with a value of '*' (see [RFC7230], Section 5.3.4).

All HTTP/2 requests MUST include exactly one valid value for the :method, :scheme, and :path pseudo-header fields, unless it is a CONNECT request (Section 8.3). An HTTP request that omits mandatory pseudo-header fields is malformed (Section 8.1.2.6).

HTTP/2 does not define a way to carry the version identifier that is included in the HTTP/1.1 request line.

8.1.2.4. Response Pseudo-Header Fields

For HTTP/2 responses, a single :status pseudo-header field is defined that carries the HTTP status codefield (see [RFC7231], Section 6). This pseudo-header field MUST be included in all responses; otherwise, theresponse is malformed (Section 8.1.2.6).

HTTP/2 does not define a way to carry the version or reason phrase that is included in an HTTP/1.1 status line.

8.1.2.5. Compressing the Cookie Header Field

The Cookie header field [COOKIE] uses a semi-colon (";") to delimit cookie-pairs (or "crumbs"). This headerfield doesn't follow the list construction rules in HTTP (see [RFC7230], Section 3.2.2), which preventscookie-pairs from being separated into different name-value pairs. This can significantly reduce compressionefficiency as individual cookie-pairs are updated.

To allow for better compression efficiency, the Cookie header field MAY be split into separate header fields,each with one or more cookie-pairs. If there are multiple Cookie header fields after decompression, theseMUST be concatenated into a single octet string using the two-octet delimiter of 0x3B, 0x20 (the ASCII string"; ") before being passed into a non-HTTP/2 context, such as an HTTP/1.1 connection, or a generic HTTPserver application.

Therefore, the following two lists of Cookie header fields are semantically equivalent.

cookie: a=b; c=d; e=f

cookie: a=b cookie: c=d cookie: e=f



8.1.2.6. Malformed Requests and Responses

A malformed request or response is one that is an otherwise valid sequence of HTTP/2 frames but is invaliddue to the presence of extraneous frames, prohibited header fields, the absence of mandatory header fields, orthe inclusion of uppercase header field names.

A request or response that includes a payload body can include a content-length header field. A requestor response is also malformed if the value of a content-length header field does not equal the sum of theDATA frame payload lengths that form the body. A response that is defined to have no payload, as describedin [RFC7230], Section 3.3.2, can have a non-zero content-length header field, even though no content isincluded in DATA frames.

Intermediaries that process HTTP requests or responses (i.e., any intermediary not acting as a tunnel) MUSTNOT forward a malformed request or response. Malformed requests or responses that are detected MUST betreated as a stream error (Section 5.4.2) of type PROTOCOL_ERROR.

For malformed requests, a server MAY send an HTTP response prior to closing or resetting the stream. ClientsMUST NOT accept a malformed response. Note that these requirements are intended to protect against severaltypes of common attacks against HTTP; they are deliberately strict because being permissive can exposeimplementations to these vulnerabilities.

8.1.3. Examples

This section shows HTTP/1.1 requests and responses, with illustrations of equivalent HTTP/2 requests andresponses.

An HTTP GET request includes request header fields and no payload body and is therefore transmitted asa single HEADERS frame, followed by zero or more CONTINUATION frames containing the serializedblock of request header fields. The HEADERS frame in the following has both the END_HEADERS andEND_STREAM flags set; no CONTINUATION frames are sent.

GET /resource HTTP/1.1 HEADERS Host: example.org ==> + END_STREAM Accept: image/jpeg + END_HEADERS :method = GET :scheme = https :path = /resource host = example.org accept = image/jpeg

Similarly, a response that includes only response header fields is transmitted as a HEADERS frame (again,followed by zero or more CONTINUATION frames) containing the serialized block of response header fields.

HTTP/1.1 304 Not Modified HEADERS ETag: "xyzzy" ==> + END_STREAM Expires: Thu, 23 Jan ... + END_HEADERS :status = 304 etag = "xyzzy" expires = Thu, 23 Jan ...

An HTTP POST request that includes request header fields and payload data is transmitted as oneHEADERS frame, followed by zero or more CONTINUATION frames containing the request header fields,followed by one or more DATA frames, with the last CONTINUATION (or HEADERS) frame having theEND_HEADERS flag set and the final DATA frame having the END_STREAM flag set:



POST /resource HTTP/1.1 HEADERS Host: example.org ==> - END_STREAM Content-Type: image/jpeg - END_HEADERS Content-Length: 123 :method = POST :path = /resource {binary data} :scheme = https

CONTINUATION + END_HEADERS content-type = image/jpeg host = example.org content-length = 123

DATA + END_STREAM {binary data}

Note that data contributing to any given header field could be spread between header block fragments. Theallocation of header fields to frames in this example is illustrative only.

A response that includes header fields and payload data is transmitted as a HEADERS frame, followed by zeroor more CONTINUATION frames, followed by one or more DATA frames, with the last DATA frame in thesequence having the END_STREAM flag set:

HTTP/1.1 200 OK HEADERS Content-Type: image/jpeg ==> - END_STREAM Content-Length: 123 + END_HEADERS :status = 200 {binary data} content-type = image/jpeg content-length = 123

DATA + END_STREAM {binary data}

An informational response using a 1xx status code other than 101 is transmitted as a HEADERS frame,followed by zero or more CONTINUATION frames.

Trailing header fields are sent as a header block after both the request or response header block and all theDATA frames have been sent. The HEADERS frame starting the trailers header block has the END_STREAMflag set.



The following example includes both a 100 (Continue) status code, which is sent in response to a requestcontaining a "100-continue" token in the Expect header field, and trailing header fields:

HTTP/1.1 100 Continue HEADERS Extension-Field: bar ==> - END_STREAM + END_HEADERS :status = 100 extension-field = bar

HTTP/1.1 200 OK HEADERS Content-Type: image/jpeg ==> - END_STREAM Transfer-Encoding: chunked + END_HEADERS Trailer: Foo :status = 200 content-length = 123 123 content-type = image/jpeg {binary data} trailer = Foo 0 Foo: bar DATA - END_STREAM {binary data}

HEADERS + END_STREAM + END_HEADERS foo = bar

8.1.4. Request Reliability Mechanisms in HTTP/2

In HTTP/1.1, an HTTP client is unable to retry a non-idempotent request when an error occurs because thereis no means to determine the nature of the error. It is possible that some server processing occurred prior to theerror, which could result in undesirable effects if the request were reattempted.

HTTP/2 provides two mechanisms for providing a guarantee to a client that a request has not been processed:

• The GOAWAY frame indicates the highest stream number that might have been processed. Requests onstreams with higher numbers are therefore guaranteed to be safe to retry.

• The REFUSED_STREAM error code can be included in a RST_STREAM frame to indicate that the streamis being closed prior to any processing having occurred. Any request that was sent on the reset stream canbe safely retried.

Requests that have not been processed have not failed; clients MAY automatically retry them, even those withnon-idempotent methods.

A server MUST NOT indicate that a stream has not been processed unless it can guarantee that fact. If framesthat are on a stream are passed to the application layer for any stream, then REFUSED_STREAM MUST NOTbe used for that stream, and a GOAWAY frame MUST include a stream identifier that is greater than or equalto the given stream identifier.

In addition to these mechanisms, the PING frame provides a way for a client to easily test a connection.Connections that remain idle can become broken as some middleboxes (for instance, network addresstranslators or load balancers) silently discard connection bindings. The PING frame allows a client to safely testwhether a connection is still active without sending a request.

8.2. Server Push

HTTP/2 allows a server to pre-emptively send (or "push") responses (along with corresponding "promised"requests) to a client in association with a previous client-initiated request. This can be useful when the serverknows the client will need to have those responses available in order to fully process the response to theoriginal request.



A client can request that server push be disabled, though this is negotiated for each hop independently. TheSETTINGS_ENABLE_PUSH setting can be set to 0 to indicate that server push is disabled.

Promised requests MUST be cacheable (see [RFC7231], Section 4.2.3), MUST be safe (see [RFC7231],Section 4.2.1), and MUST NOT include a request body. Clients that receive a promised request that is notcacheable, that is not known to be safe, or that indicates the presence of a request body MUST reset thepromised stream with a stream error (Section 5.4.2) of type PROTOCOL_ERROR. Note this could result in thepromised stream being reset if the client does not recognize a newly defined method as being safe.

Pushed responses that are cacheable (see [RFC7234], Section 3) can be stored by the client, if it implementsan HTTP cache. Pushed responses are considered successfully validated on the origin server (e.g., if the "no-cache" cache response directive is present ([RFC7234], Section 5.2.2)) while the stream identified by thepromised stream ID is still open.

Pushed responses that are not cacheable MUST NOT be stored by any HTTP cache. They MAY be madeavailable to the application separately.

The server MUST include a value in the :authority pseudo-header field for which the server isauthoritative (see Section 10.1). A client MUST treat a PUSH_PROMISE for which the server is notauthoritative as a stream error (Section 5.4.2) of type PROTOCOL_ERROR.

An intermediary can receive pushes from the server and choose not to forward them on to the client. In otherwords, how to make use of the pushed information is up to that intermediary. Equally, the intermediary mightchoose to make additional pushes to the client, without any action taken by the server.

A client cannot push. Thus, servers MUST treat the receipt of a PUSH_PROMISE frame as a connectionerror (Section 5.4.1) of type PROTOCOL_ERROR. Clients MUST reject any attempt to change theSETTINGS_ENABLE_PUSH setting to a value other than 0 by treating the message as a connection error(Section 5.4.1) of type PROTOCOL_ERROR.

8.2.1. Push Requests

Server push is semantically equivalent to a server responding to a request; however, in this case, that request isalso sent by the server, as a PUSH_PROMISE frame.

The PUSH_PROMISE frame includes a header block that contains a complete set of request header fields thatthe server attributes to the request. It is not possible to push a response to a request that includes a request body.

Pushed responses are always associated with an explicit request from the client. The PUSH_PROMISE framessent by the server are sent on that explicit request's stream. The PUSH_PROMISE frame also includes apromised stream identifier, chosen from the stream identifiers available to the server (see Section 5.1.1).

The header fields in PUSH_PROMISE and any subsequent CONTINUATION frames MUST be a valid andcomplete set of request header fields (Section 8.1.2.3). The server MUST include a method in the :methodpseudo-header field that is safe and cacheable. If a client receives a PUSH_PROMISE that does not includea complete and valid set of header fields or the :method pseudo-header field identifies a method that is notsafe, it MUST respond with a stream error (Section 5.4.2) of type PROTOCOL_ERROR.

The server SHOULD send PUSH_PROMISE (Section 6.6) frames prior to sending any frames thatreference the promised responses. This avoids a race where clients issue requests prior to receiving anyPUSH_PROMISE frames.

For example, if the server receives a request for a document containing embedded links to multiple imagefiles and the server chooses to push those additional images to the client, sending PUSH_PROMISE framesbefore the DATA frames that contain the image links ensures that the client is able to see that a resource willbe pushed before discovering embedded links. Similarly, if the server pushes responses referenced by theheader block (for instance, in Link header fields), sending a PUSH_PROMISE before sending the header blockensures that clients do not request those resources.

PUSH_PROMISE frames MUST NOT be sent by the client.



PUSH_PROMISE frames can be sent by the server in response to any client-initiated stream, but the streamMUST be in either the "open" or "half-closed (remote)" state with respect to the server. PUSH_PROMISEframes are interspersed with the frames that comprise a response, though they cannot be interspersed withHEADERS and CONTINUATION frames that comprise a single header block.

Sending a PUSH_PROMISE frame creates a new stream and puts the stream into the “reserved (local)” statefor the server and the “reserved (remote)” state for the client.

8.2.2. Push Responses

After sending the PUSH_PROMISE frame, the server can begin delivering the pushed response as a response(Section 8.1.2.4) on a server-initiated stream that uses the promised stream identifier. The server uses thisstream to transmit an HTTP response, using the same sequence of frames as defined in Section 8.1. This streambecomes "half-closed" to the client (Section 5.1) after the initial HEADERS frame is sent.

Once a client receives a PUSH_PROMISE frame and chooses to accept the pushed response, the clientSHOULD NOT issue any requests for the promised response until after the promised stream has closed.

If the client determines, for any reason, that it does not wish to receive the pushed response from the server orif the server takes too long to begin sending the promised response, the client can send a RST_STREAM frame,using either the CANCEL or REFUSED_STREAM code and referencing the pushed stream's identifier.

A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit the number of responsesthat can be concurrently pushed by a server. Advertising a SETTINGS_MAX_CONCURRENT_STREAMSvalue of zero disables server push by preventing the server from creating the necessary streams. This does notprohibit a server from sending PUSH_PROMISE frames; clients need to reset any promised streams that arenot wanted.

Clients receiving a pushed response MUST validate that either the server is authoritative (see Section 10.1)or the proxy that provided the pushed response is configured for the corresponding request. For example, aserver that offers a certificate for only the example.com DNS-ID or Common Name is not permitted to pusha response for https://www.example.org/doc.

The response for a PUSH_PROMISE stream begins with a HEADERS frame, which immediately puts thestream into the "half-closed (remote)" state for the server and "half-closed (local)" state for the client, and endswith a frame bearing END_STREAM, which places the stream in the "closed" state.

Note: The client never sends a frame with the END_STREAM flag for a server push.

8.3. The CONNECT Method

In HTTP/1.x, the pseudo-method CONNECT ([RFC7231], Section 4.3.6) is used to convert an HTTPconnection into a tunnel to a remote host. CONNECT is primarily used with HTTP proxies to establish a TLSsession with an origin server for the purposes of interacting with https resources.

In HTTP/2, the CONNECT method is used to establish a tunnel over a single HTTP/2 stream to a remote hostfor similar purposes. The HTTP header field mapping works as defined in Section 8.1.2.3 ("Request Pseudo-Header Fields"), with a few differences. Specifically:

• The :method pseudo-header field is set to CONNECT.

• The :scheme and :path pseudo-header fields MUST be omitted.

• The :authority pseudo-header field contains the host and port to connect to (equivalent to theauthority-form of the request-target of CONNECT requests (see [RFC7230], Section 5.3)).

A CONNECT request that does not conform to these restrictions is malformed (Section 8.1.2.6).

A proxy that supports CONNECT establishes a TCP connection [TCP] to the server identified in the:authority pseudo-header field. Once this connection is successfully established, the proxy sends aHEADERS frame containing a 2xx series status code to the client, as defined in [RFC7231], Section 4.3.6.



After the initial HEADERS frame sent by each peer, all subsequent DATA frames correspond to data sent onthe TCP connection. The payload of any DATA frames sent by the client is transmitted by the proxy to theTCP server; data received from the TCP server is assembled into DATA frames by the proxy. Frame typesother than DATA or stream management frames (RST_STREAM, WINDOW_UPDATE, and PRIORITY)MUST NOT be sent on a connected stream and MUST be treated as a stream error (Section 5.4.2) if received.

The TCP connection can be closed by either peer. The END_STREAM flag on a DATA frame is treated asbeing equivalent to the TCP FIN bit. A client is expected to send a DATA frame with the END_STREAM flagset after receiving a frame bearing the END_STREAM flag. A proxy that receives a DATA frame with theEND_STREAM flag set sends the attached data with the FIN bit set on the last TCP segment. A proxy thatreceives a TCP segment with the FIN bit set sends a DATA frame with the END_STREAM flag set. Note thatthe final TCP segment or DATA frame could be empty.

A TCP connection error is signaled with RST_STREAM. A proxy treats any error in the TCP connection,which includes receiving a TCP segment with the RST bit set, as a stream error (Section 5.4.2) of typeCONNECT_ERROR. Correspondingly, a proxy MUST send a TCP segment with the RST bit set if it detectsan error with the stream or the HTTP/2 connection.



9. Additional HTTP Requirements/Considerations

This section outlines attributes of the HTTP protocol that improve interoperability, reduce exposure to knownsecurity vulnerabilities, or reduce the potential for implementation variation.

9.1. Connection Management

HTTP/2 connections are persistent. For best performance, it is expected that clients will not close connectionsuntil it is determined that no further communication with a server is necessary (for example, when a usernavigates away from a particular web page) or until the server closes the connection.

Clients SHOULD NOT open more than one HTTP/2 connection to a given host and port pair, where the host isderived from a URI, a selected alternative service [ALT-SVC], or a configured proxy.

A client can create additional connections as replacements, either to replace connections that are near toexhausting the available stream identifier space (Section 5.1.1), to refresh the keying material for a TLSconnection, or to replace connections that have encountered errors (Section 5.4.1).

A client MAY open multiple connections to the same IP address and TCP port using different Server NameIndication [TLS-EXT] values or to provide different TLS client certificates but SHOULD avoid creatingmultiple connections with the same configuration.

Servers are encouraged to maintain open connections for as long as possible but are permitted to terminateidle connections if necessary. When either endpoint chooses to close the transport-layer TCP connection, theterminating endpoint SHOULD first send a GOAWAY (Section 6.8) frame so that both endpoints can reliablydetermine whether previously sent frames have been processed and gracefully complete or terminate anynecessary remaining tasks.

9.1.1. Connection Reuse

Connections that are made to an origin server, either directly or through a tunnel created using the CONNECTmethod (Section 8.3), MAY be reused for requests with multiple different URI authority components. Aconnection can be reused as long as the origin server is authoritative (Section 10.1). For TCP connectionswithout TLS, this depends on the host having resolved to the same IP address.

For https resources, connection reuse additionally depends on having a certificate that is valid for the host inthe URI. The certificate presented by the server MUST satisfy any checks that the client would perform whenforming a new TLS connection for the host in the URI.

An origin server might offer a certificate with multiple subjectAltName attributes or nameswith wildcards, one of which is valid for the authority in the URI. For example, a certificate with asubjectAltName of *.example.com might permit the use of the same connection for requests to URIsstarting with https://a.example.com/ and https://b.example.com/.

In some deployments, reusing a connection for multiple origins can result in requests being directed to thewrong origin server. For example, TLS termination might be performed by a middlebox that uses the TLSServer Name Indication (SNI) [TLS-EXT] extension to select an origin server. This means that it is possiblefor clients to send confidential information to servers that might not be the intended target for the request, eventhough the server is otherwise authoritative.

A server that does not wish clients to reuse connections can indicate that it is not authoritative for a request bysending a 421 (Misdirected Request) status code in response to the request (see Section 9.1.2).

A client that is configured to use a proxy over HTTP/2 directs requests to that proxy through a singleconnection. That is, all requests sent via a proxy reuse the connection to the proxy.



9.1.2. The 421 (Misdirected Request) Status Code

The 421 (Misdirected Request) status code indicates that the request was directed at a server that is notable to produce a response. This can be sent by a server that is not configured to produce responses for thecombination of scheme and authority that are included in the request URI.

Clients receiving a 421 (Misdirected Request) response from a server MAY retry the request — whether therequest method is idempotent or not — over a different connection. This is possible if a connection is reused(Section 9.1.1) or if an alternative service is selected [ALT-SVC].

This status code MUST NOT be generated by proxies.

A 421 response is cacheable by default, i.e., unless otherwise indicated by the method definition or explicitcache controls (see Section 4.2.2 of [RFC7234]).

9.2. Use of TLS Features

Implementations of HTTP/2 MUST use TLS version 1.2 [TLS12] or higher for HTTP/2 over TLS. The generalTLS usage guidance in [TLSBCP] SHOULD be followed, with some additional restrictions that are specific toHTTP/2.

The TLS implementation MUST support the Server Name Indication (SNI) [TLS-EXT] extension to TLS.HTTP/2 clients MUST indicate the target domain name when negotiating TLS.

Deployments of HTTP/2 that negotiate TLS 1.3 or higher need only support and use the SNI extension;deployments of TLS 1.2 are subject to the requirements in the following sections. Implementations areencouraged to provide defaults that comply, but it is recognized that deployments are ultimately responsible forcompliance.

9.2.1. TLS 1.2 Features

This section describes restrictions on the TLS 1.2 feature set that can be used with HTTP/2. Due to deploymentlimitations, it might not be possible to fail TLS negotiation when these restrictions are not met. An endpointMAY immediately terminate an HTTP/2 connection that does not meet these TLS requirements with aconnection error (Section 5.4.1) of type INADEQUATE_SECURITY.

A deployment of HTTP/2 over TLS 1.2 MUST disable compression. TLS compression can lead to theexposure of information that would not otherwise be revealed [RFC3749]. Generic compression is unnecessarysince HTTP/2 provides compression features that are more aware of context and therefore likely to be moreappropriate for use for performance, security, or other reasons.

A deployment of HTTP/2 over TLS 1.2 MUST disable renegotiation. An endpoint MUST treat a TLSrenegotiation as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Note that disablingrenegotiation can result in long-lived connections becoming unusable due to limits on the number of messagesthe underlying cipher suite can encipher.

An endpoint MAY use renegotiation to provide confidentiality protection for client credentials offered in thehandshake, but any renegotiation MUST occur prior to sending the connection preface. A server SHOULDrequest a client certificate if it sees a renegotiation request immediately after establishing a connection.

This effectively prevents the use of renegotiation in response to a request for a specific protected resource. Afuture specification might provide a way to support this use case. Alternatively, a server might use an error(Section 5.4) of type HTTP_1_1_REQUIRED to request the client use a protocol that supports renegotiation.

Implementations MUST support ephemeral key exchange sizes of at least 2048 bits for cipher suites that useephemeral finite field Diffie-Hellman (DHE) [TLS12] and 224 bits for cipher suites that use ephemeral ellipticcurve Diffie-Hellman (ECDHE) [RFC4492]. Clients MUST accept DHE sizes of up to 4096 bits. EndpointsMAY treat negotiation of key sizes smaller than the lower limits as a connection error (Section 5.4.1) of typeINADEQUATE_SECURITY.



9.2.2. TLS 1.2 Cipher Suites

A deployment of HTTP/2 over TLS 1.2 SHOULD NOT use any of the cipher suites that are listed in the ciphersuite black list (Appendix A).

Endpoints MAY choose to generate a connection error (Section 5.4.1) of type INADEQUATE_SECURITY ifone of the cipher suites from the black list is negotiated. A deployment that chooses to use a black-listed ciphersuite risks triggering a connection error unless the set of potential peers is known to accept that cipher suite.

Implementations MUST NOT generate this error in reaction to the negotiation of a cipher suite that is noton the black list. Consequently, when clients offer a cipher suite that is not on the black list, they have to beprepared to use that cipher suite with HTTP/2.

The black list includes the cipher suite that TLS 1.2 makes mandatory, which means that TLS 1.2deployments could have non-intersecting sets of permitted cipher suites. To avoid this problemcausing TLS handshake failures, deployments of HTTP/2 that use TLS 1.2 MUST supportTLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 [TLS-ECDHE] with the P-256 elliptic curve[FIPS186].

Note that clients might advertise support of cipher suites that are on the black list in order to allow forconnection to servers that do not support HTTP/2. This allows servers to select HTTP/1.1 with a cipher suitethat is on the HTTP/2 black list. However, this can result in HTTP/2 being negotiated with a black-listed ciphersuite if the application protocol and cipher suite are independently selected.



10. Security Considerations

10.1. Server Authority

HTTP/2 relies on the HTTP/1.1 definition of authority for determining whether a server is authoritative inproviding a given response (see [RFC7230], Section 9.1). This relies on local name resolution for the "http"URI scheme and the authenticated server identity for the "https" scheme (see [RFC2818], Section 3).

10.2. Cross-Protocol Attacks

In a cross-protocol attack, an attacker causes a client to initiate a transaction in one protocol toward a serverthat understands a different protocol. An attacker might be able to cause the transaction to appear as a validtransaction in the second protocol. In combination with the capabilities of the web context, this can be used tointeract with poorly protected servers in private networks.

Completing a TLS handshake with an ALPN identifier for HTTP/2 can be considered sufficient protectionagainst cross-protocol attacks. ALPN provides a positive indication that a server is willing to proceed withHTTP/2, which prevents attacks on other TLS-based protocols.

The encryption in TLS makes it difficult for attackers to control the data that could be used in a cross-protocolattack on a cleartext protocol.

The cleartext version of HTTP/2 has minimal protection against cross-protocol attacks. The connection preface(Section 3.5) contains a string that is designed to confuse HTTP/1.1 servers, but no special protection is offeredfor other protocols. A server that is willing to ignore parts of an HTTP/1.1 request containing an Upgradeheader field in addition to the client connection preface could be exposed to a cross-protocol attack.

10.3. Intermediary Encapsulation Attacks

The HTTP/2 header field encoding allows the expression of names that are not valid field names in the InternetMessage Syntax used by HTTP/1.1. Requests or responses containing invalid header field names MUSTbe treated as malformed (Section 8.1.2.6). An intermediary therefore cannot translate an HTTP/2 request orresponse containing an invalid field name into an HTTP/1.1 message.

Similarly, HTTP/2 allows header field values that are not valid. While most of the values that can be encodedwill not alter header field parsing, carriage return (CR, ASCII 0xd), line feed (LF, ASCII 0xa), and the zerocharacter (NUL, ASCII 0x0) might be exploited by an attacker if they are translated verbatim. Any requestor response that contains a character not permitted in a header field value MUST be treated as malformed(Section 8.1.2.6). Valid characters are defined by the field-content ABNF rule in Section 3.2 of[RFC7230].

10.4. Cacheability of Pushed Responses

Pushed responses do not have an explicit request from the client; the request is provided by the server in thePUSH_PROMISE frame.

Caching responses that are pushed is possible based on the guidance provided by the origin server in the Cache-Control header field. However, this can cause issues if a single server hosts more than one tenant. For example,a server might offer multiple users each a small portion of its URI space.

Where multiple tenants share space on the same server, that server MUST ensure that tenants are not able topush representations of resources that they do not have authority over. Failure to enforce this would allow atenant to provide a representation that would be served out of cache, overriding the actual representation thatthe authoritative tenant provides.

Pushed responses for which an origin server is not authoritative (see Section 10.1) MUST NOT be used orcached.



10.5. Denial-of-Service Considerations

An HTTP/2 connection can demand a greater commitment of resources to operate than an HTTP/1.1connection. The use of header compression and flow control depend on a commitment of resources for storinga greater amount of state. Settings for these features ensure that memory commitments for these features arestrictly bounded.

The number of PUSH_PROMISE frames is not constrained in the same fashion. A client that acceptsserver push SHOULD limit the number of streams it allows to be in the "reserved (remote)" state.An excessive number of server push streams can be treated as a stream error (Section 5.4.2) of typeENHANCE_YOUR_CALM.

Processing capacity cannot be guarded as effectively as state capacity.

The SETTINGS frame can be abused to cause a peer to expend additional processing time. This might be doneby pointlessly changing SETTINGS parameters, setting multiple undefined parameters, or changing the samesetting multiple times in the same frame. WINDOW_UPDATE or PRIORITY frames can be abused to causean unnecessary waste of resources.

Large numbers of small or empty frames can be abused to cause a peer to expend time processing frameheaders. Note, however, that some uses are entirely legitimate, such as the sending of an empty DATA orCONTINUATION frame at the end of a stream.

Header compression also offers some opportunities to waste processing resources; see Section 7 of[COMPRESSION] for more details on potential abuses.

Limits in SETTINGS parameters cannot be reduced instantaneously, which leaves an endpoint exposedto behavior from a peer that could exceed the new limits. In particular, immediately after establishing aconnection, limits set by a server are not known to clients and could be exceeded without being an obviousprotocol violation.

All these features — i.e., SETTINGS changes, small frames, header compression — have legitimate uses.These features become a burden only when they are used unnecessarily or to excess.

An endpoint that doesn't monitor this behavior exposes itself to a risk of denial-of-service attack.Implementations SHOULD track the use of these features and set limits on their use. An endpoint MAY treatactivity that is suspicious as a connection error (Section 5.4.1) of type ENHANCE_YOUR_CALM.

10.5.1. Limits on Header Block Size

A large header block (Section 4.3) can cause an implementation to commit a large amount of state. Headerfields that are critical for routing can appear toward the end of a header block, which prevents streaming ofheader fields to their ultimate destination. This ordering and other reasons, such as ensuring cache correctness,mean that an endpoint might need to buffer the entire header block. Since there is no hard limit to the size of aheader block, some endpoints could be forced to commit a large amount of available memory for header fields.

An endpoint can use the SETTINGS_MAX_HEADER_LIST_SIZE to advise peers of limits that might applyon the size of header blocks. This setting is only advisory, so endpoints MAY choose to send header blocks thatexceed this limit and risk having the request or response being treated as malformed. This setting is specific toa connection, so any request or response could encounter a hop with a lower, unknown limit. An intermediarycan attempt to avoid this problem by passing on values presented by different peers, but they are not obligatedto do so.

A server that receives a larger header block than it is willing to handle can send an HTTP 431 (Request HeaderFields Too Large) status code [RFC6585]. A client can discard responses that it cannot process. The headerblock MUST be processed to ensure a consistent connection state, unless the connection is closed.

10.5.2. CONNECT Issues

The CONNECT method can be used to create disproportionate load on an proxy, since stream creationis relatively inexpensive when compared to the creation and maintenance of a TCP connection. A proxy



might also maintain some resources for a TCP connection beyond the closing of the stream that carries theCONNECT request, since the outgoing TCP connection remains in the TIME_WAIT state. Therefore, a proxycannot rely on SETTINGS_MAX_CONCURRENT_STREAMS alone to limit the resources consumed byCONNECT requests.

10.6. Use of Compression

Compression can allow an attacker to recover secret data when it is compressed in the same context as dataunder attacker control. HTTP/2 enables compression of header fields (Section 4.3); the following concerns alsoapply to the use of HTTP compressed content-codings ([RFC7231], Section 3.1.2.1).

There are demonstrable attacks on compression that exploit the characteristics of the web (e.g., [BREACH]).The attacker induces multiple requests containing varying plaintext, observing the length of the resultingciphertext in each, which reveals a shorter length when a guess about the secret is correct.

Implementations communicating on a secure channel MUST NOT compress content that includes bothconfidential and attacker-controlled data unless separate compression dictionaries are used for each source ofdata. Compression MUST NOT be used if the source of data cannot be reliably determined. Generic streamcompression, such as that provided by TLS, MUST NOT be used with HTTP/2 (see Section 9.2).

Further considerations regarding the compression of header fields are described in [COMPRESSION].

10.7. Use of Padding

Padding within HTTP/2 is not intended as a replacement for general purpose padding, such as might beprovided by TLS [TLS12]. Redundant padding could even be counterproductive. Correct application candepend on having specific knowledge of the data that is being padded.

To mitigate attacks that rely on compression, disabling or limiting compression might be preferable to paddingas a countermeasure.

Padding can be used to obscure the exact size of frame content and is provided to mitigate specific attackswithin HTTP, for example, attacks where compressed content includes both attacker-controlled plaintext andsecret data (e.g., [BREACH]).

Use of padding can result in less protection than might seem immediately obvious. At best, padding only makesit more difficult for an attacker to infer length information by increasing the number of frames an attackerhas to observe. Incorrectly implemented padding schemes can be easily defeated. In particular, randomizedpadding with a predictable distribution provides very little protection; similarly, padding payloads to a fixedsize exposes information as payload sizes cross the fixed-sized boundary, which could be possible if an attackercan control plaintext.

Intermediaries SHOULD retain padding for DATA frames but MAY drop padding for HEADERS andPUSH_PROMISE frames. A valid reason for an intermediary to change the amount of padding of frames is toimprove the protections that padding provides.

10.8. Privacy Considerations

Several characteristics of HTTP/2 provide an observer an opportunity to correlate actions of a single client orserver over time. These include the value of settings, the manner in which flow-control windows are managed,the way priorities are allocated to streams, the timing of reactions to stimulus, and the handling of any featuresthat are controlled by settings.

As far as these create observable differences in behavior, they could be used as a basis for fingerprinting aspecific client, as defined in Section 1.8 of [HTML5].

HTTP/2's preference for using a single TCP connection allows correlation of a user's activity on a site. Reusingconnections for different origins allows tracking across those origins.



Because the PING and SETTINGS frames solicit immediate responses, they can be used by an endpoint tomeasure latency to their peer. This might have privacy implications in certain scenarios.



11. IANA Considerations

A string for identifying HTTP/2 is entered into the "Application-Layer Protocol Negotiation (ALPN) ProtocolIDs" registry established in [TLS-ALPN].

This document establishes a registry for frame types, settings, and error codes. These new registries appear inthe new "Hypertext Transfer Protocol version 2 (HTTP/2) Parameters" section.

This document registers the HTTP2-Settings header field for use in HTTP; it also registers the 421(Misdirected Request) status code.

This document registers the PRI method for use in HTTP to avoid collisions with the connection preface(Section 3.5).

11.1. Registration of HTTP/2 Identification Strings

This document creates two registrations for the identification of HTTP/2 (see Section 3.3) in the "Application-Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in [TLS-ALPN].

The "h2" string identifies HTTP/2 when used over TLS:

Protocol: HTTP/2 over TLS

Identification Sequence: 0x68 0x32 ("h2")

Specification: This document

The "h2c" string identifies HTTP/2 when used over cleartext TCP:

Protocol: HTTP/2 over TCP

Identification Sequence: 0x68 0x32 0x63 ("h2c")

Specification: This document

11.2. Frame Type Registry

This document establishes a registry for HTTP/2 frame type codes. The "HTTP/2 Frame Type" registrymanages an 8-bit space. The "HTTP/2 Frame Type" registry operates under either of the "IETF Review" or"IESG Approval" policies [RFC5226] for values between 0x00 and 0xef, with values between 0xf0 and 0xffbeing reserved for Experimental Use.

New entries in this registry require the following information:

Frame Type: A name or label for the frame type.

Code: The 8-bit code assigned to the frame type.

Specification: A reference to a specification that includes a description of the frame layout, itssemantics, and flags that the frame type uses, including any parts of the frame thatare conditionally present based on the value of flags.

The entries in the following table are registered by this document.

Frame Type Code SectionDATA 0x0 Section 6.1HEADERS 0x1 Section 6.2PRIORITY 0x2 Section 6.3RST_STREAM 0x3 Section 6.4SETTINGS 0x4 Section 6.5PUSH_PROMISE 0x5 Section 6.6PING 0x6 Section 6.7GOAWAY 0x7 Section 6.8WINDOW_UPDATE 0x8 Section 6.9



Frame Type Code SectionCONTINUATION 0x9 Section 6.10

11.3. Settings Registry

This document establishes a registry for HTTP/2 settings. The "HTTP/2 Settings" registry manages a 16-bitspace. The "HTTP/2 Settings" registry operates under the "Expert Review" policy [RFC5226] for values in therange from 0x0000 to 0xefff, with values between and 0xf000 and 0xffff being reserved for Experimental Use.

New registrations are advised to provide the following information:

Name: A symbolic name for the setting. Specifying a setting name is optional.

Code: The 16-bit code assigned to the setting.

Initial Value: An initial value for the setting.

Specification: An optional reference to a specification that describes the use of the setting.


Name Code Initial Value SpecificationHEADER_TABLE_SIZE 0x1 4096 Section 6.5.2ENABLE_PUSH 0x2 1 Section 6.5.2MAX_CONCURRENT_STREAMS0x3 (infinite) Section 6.5.2INITIAL_WINDOW_SIZE0x4 65535 Section 6.5.2MAX_FRAME_SIZE 0x5 16384 Section 6.5.2MAX_HEADER_LIST_SIZE0x6 (infinite) Section 6.5.2

11.4. Error Code Registry

This document establishes a registry for HTTP/2 error codes. The "HTTP/2 Error Code" registry manages a 32-bit space. The "HTTP/2 Error Code" registry operates under the "Expert Review" policy [RFC5226].

Registrations for error codes are required to include a description of the error code. An expert reviewer isadvised to examine new registrations for possible duplication with existing error codes. Use of existingregistrations is to be encouraged, but not mandated.

New registrations are advised to provide the following information:

Name: A name for the error code. Specifying an error code name is optional.

Code: The 32-bit error code value.

Description: A brief description of the error code semantics, longer if no detailed specification isprovided.

Specification: An optional reference for a specification that defines the error code.


Name Code Description SpecificationNO_ERROR 0x0 Graceful shutdown Section 7PROTOCOL_ERROR 0x1 Protocol error detected Section 7INTERNAL_ERROR 0x2 Implementation fault Section 7FLOW_CONTROL_ERROR0x3 Flow-control limits

exceededSection 7

SETTINGS_TIMEOUT 0x4 Settings notacknowledged

Section 7

STREAM_CLOSED 0x5 Frame received forclosed stream

Section 7

FRAME_SIZE_ERROR 0x6 Frame size incorrect Section 7REFUSED_STREAM 0x7 Stream not processed Section 7



Name Code Description SpecificationCANCEL 0x8 Stream cancelled Section 7COMPRESSION_ERROR 0x9 Compression state not

updatedSection 7

CONNECT_ERROR 0xa TCP connection error forCONNECT method

Section 7

ENHANCE_YOUR_CALM0xb Processing capacityexceeded

Section 7

INADEQUATE_SECURITY0xc Negotiated TLSparameters notacceptable

Section 7

HTTP_1_1_REQUIRED 0xd Use HTTP/1.1 for therequest

Section 7

11.5. HTTP2-Settings Header Field Registration

This section registers the HTTP2-Settings header field in the "Permanent Message Header Field Names"registry [BCP90].

Header field name: HTTP2-Settings

Applicable protocol: http

Status: standard

Author/Change controller: IETF

Specification document(s): Section 3.2.1 of this document

Related information: This header field is only used by an HTTP/2 client forUpgrade-based negotiation.

11.6. PRI Method Registration

This section registers the PRI method in the "HTTP Method Registry" ([RFC7231], Section 8.1).

Method Name: PRI

Safe: Yes

Idempotent: Yes

Specification document(s): Section 3.5 of this document

Related information: This method is never used by an actual client. This methodwill appear to be used when an HTTP/1.1 server orintermediary attempts to parse an HTTP/2 connectionpreface.

11.7. The 421 (Misdirected Request) HTTP Status Code

This document registers the 421 (Misdirected Request) HTTP status code in the "HTTP Status Codes" registry([RFC7231], Section 8.2).

Status Code: 421

Short Description: Misdirected Request

Specification: Section 9.1.2 of this document



11.8. The h2c Upgrade Token

This document registers the "h2c" upgrade token in the "HTTP Upgrade Tokens" registry ([RFC7230], Section8.6).

Value: h2c

Description: Hypertext Transfer Protocol version 2 (HTTP/2)

Expected Version Tokens: None

Reference: Section 3.2 of this document



12. References

12.1. Normative References

[COMPRESSION] Peon, R. and H. Ruellan, "HPACK: Header Compression for HTTP/2", RFC 7541,DOI 10.17487/RFC7541, May 2015, <http://www.rfc-editor.org/info/rfc>.

[COOKIE] Barth, A., "HTTP State Management Mechanism", RFC 6265, DOI 10.17487/RFC6265,April 2011, <http://www.rfc-editor.org/info/rfc>.

[FIPS186] NIST, "Digital Signature Standard (DSS)", FIPS PUB 186-4, July 2013, <http://dx.doi.org/10.6028/NIST.FIPS.186-4>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14,RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc>.

[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/RFC2818, May 2000, <http://www.rfc-editor.org/info/rfc>.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): GenericSyntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, <http://www.rfc-editor.org/info/rfc>.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, <http://www.rfc-editor.org/info/rfc>.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section inRFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF",STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <http://www.rfc-editor.org/info/rfc>.

[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): MessageSyntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014, <http://www.rfc-editor.org/info/rfc>.

[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semanticsand Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, <http://www.rfc-editor.org/info/rfc>.

[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): ConditionalRequests", RFC 7232, DOI 10.17487/RFC7232, June 2014, <http://www.rfc-editor.org/info/rfc>.

[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol(HTTP/1.1): Range Requests", RFC 7233, DOI 10.17487/RFC7233, June 2014, <http://www.rfc-editor.org/info/rfc>.

[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol(HTTP/1.1): Caching", RFC 7234, DOI 10.17487/RFC7234, June 2014, <http://www.rfc-editor.org/info/rfc>.

[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1):Authentication", RFC 7235, DOI 10.17487/RFC7235, June 2014, <http://www.rfc-editor.org/info/rfc>.

[TCP] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793,September 1981, <http://www.rfc-editor.org/info/rfc>.


https://tools.ietf.org/html/rfc7541

http://dx.doi.org/10.17487/RFC7541



http://dx.doi.org/10.6028/NIST.FIPS.186-4





































[TLS-ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan, "Transport Layer Security (TLS)Application-Layer Protocol Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,July 2014, <http://www.rfc-editor.org/info/rfc>.

[TLS-ECDHE] Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois CounterMode (GCM)", RFC 5289, DOI 10.17487/RFC5289, August 2008, <http://www.rfc-editor.org/info/rfc>.

[TLS-EXT] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions",RFC 6066, DOI 10.17487/RFC6066, January 2011, <http://www.rfc-editor.org/info/rfc>.

[TLS12] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2",RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc>.

12.2. Informative References

[ALT-SVC] Nottingham, M., McManus, P., and J. Reschke, "HTTP Alternative Services", Work inProgress, draft-ietf-httpbis-alt-svc-06, February 2015.

[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message HeaderFields", BCP 90, RFC 3864, September 2004, <http://www.rfc-editor.org/info/bcp>.

[BREACH] Gluck, Y., Harris, N., and A. Prado, "BREACH: Reviving the CRIME Attack", July 2013,<http://breachattack.com/resources/BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf>.

[HTML5] Hickson, I., Berjon, R., Faulkner, S., Leithead, T., Doyle Navara, E., O'Connor, E., and S.Pfeiffer, "HTML5", W3C Recommendation REC-html5-20141028, October 2014, <http://www.w3.org/TR/2014/REC-html5-20141028/>.

[RFC3749] Hollenbeck, S., "Transport Layer Security Protocol Compression Methods", RFC 3749,DOI 10.17487/RFC3749, May 2004, <http://www.rfc-editor.org/info/rfc>.

[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. Moeller, "Elliptic CurveCryptography (ECC) Cipher Suites for Transport Layer Security (TLS)", RFC 4492,DOI 10.17487/RFC4492, May 2006, <http://www.rfc-editor.org/info/rfc>.

[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012, <http://www.rfc-editor.org/info/rfc>.

[RFC7323] Borman, D., Braden, B., Jacobson, V., and R. Scheffenegger, Ed., "TCP Extensions for HighPerformance", RFC 7323, DOI 10.17487/RFC7323, September 2014, <http://www.rfc-editor.org/info/rfc>.

[TALKING] Huang, L., Chen, E., Barth, A., Rescorla, E., and C. Jackson, "Talking to Yourself for Fun andProfit", 2011, <http://w2spconf.com/2011/papers/websocket.pdf>.

[TLSBCP] Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations for Secure Use of TransportLayer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525,DOI 10.17487/RFC7525, May 2015, <http://www.rfc-editor.org/info/rfc>.












https://tools.ietf.org/html/draft-ietf-httpbis-alt-svc-06



http://breachattack.com/resources/BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf

http://www.w3.org/TR/2014/REC-html5-20141028/












http://w2spconf.com/2011/papers/websocket.pdf

http://w2spconf.com/2011/papers/websocket.pdf





Appendix A. TLS 1.2 Cipher Suite Black List

An HTTP/2 implementation MAY treat the negotiation of any of the following cipher suites with TLS 1.2 as aconnection error (Section 5.4.1) of type INADEQUATE_SECURITY:

• TLS_NULL_WITH_NULL_NULL

• TLS_RSA_WITH_NULL_MD5

• TLS_RSA_WITH_NULL_SHA

• TLS_RSA_EXPORT_WITH_RC4_40_MD5

• TLS_RSA_WITH_RC4_128_MD5

• TLS_RSA_WITH_RC4_128_SHA

• TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5

• TLS_RSA_WITH_IDEA_CBC_SHA

• TLS_RSA_EXPORT_WITH_DES40_CBC_SHA

• TLS_RSA_WITH_DES_CBC_SHA

• TLS_RSA_WITH_3DES_EDE_CBC_SHA

• TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA

• TLS_DH_DSS_WITH_DES_CBC_SHA

• TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA

• TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA

• TLS_DH_RSA_WITH_DES_CBC_SHA

• TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA

• TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA

• TLS_DHE_DSS_WITH_DES_CBC_SHA

• TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA

• TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA

• TLS_DHE_RSA_WITH_DES_CBC_SHA

• TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA

• TLS_DH_anon_EXPORT_WITH_RC4_40_MD5

• TLS_DH_anon_WITH_RC4_128_MD5

• TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA

• TLS_DH_anon_WITH_DES_CBC_SHA

• TLS_DH_anon_WITH_3DES_EDE_CBC_SHA

• TLS_KRB5_WITH_DES_CBC_SHA

• TLS_KRB5_WITH_3DES_EDE_CBC_SHA

• TLS_KRB5_WITH_RC4_128_SHA

• TLS_KRB5_WITH_IDEA_CBC_SHA

• TLS_KRB5_WITH_DES_CBC_MD5

• TLS_KRB5_WITH_3DES_EDE_CBC_MD5

• TLS_KRB5_WITH_RC4_128_MD5

• TLS_KRB5_WITH_IDEA_CBC_MD5

• TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA

• TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA

• TLS_KRB5_EXPORT_WITH_RC4_40_SHA

• TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5

• TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5



• TLS_KRB5_EXPORT_WITH_RC4_40_MD5

• TLS_PSK_WITH_NULL_SHA

• TLS_DHE_PSK_WITH_NULL_SHA

• TLS_RSA_PSK_WITH_NULL_SHA

• TLS_RSA_WITH_AES_128_CBC_SHA

• TLS_DH_DSS_WITH_AES_128_CBC_SHA

• TLS_DH_RSA_WITH_AES_128_CBC_SHA

• TLS_DHE_DSS_WITH_AES_128_CBC_SHA

• TLS_DHE_RSA_WITH_AES_128_CBC_SHA

• TLS_DH_anon_WITH_AES_128_CBC_SHA

• TLS_RSA_WITH_AES_256_CBC_SHA

• TLS_DH_DSS_WITH_AES_256_CBC_SHA

• TLS_DH_RSA_WITH_AES_256_CBC_SHA

• TLS_DHE_DSS_WITH_AES_256_CBC_SHA

• TLS_DHE_RSA_WITH_AES_256_CBC_SHA

• TLS_DH_anon_WITH_AES_256_CBC_SHA

• TLS_RSA_WITH_NULL_SHA256

• TLS_RSA_WITH_AES_128_CBC_SHA256

• TLS_RSA_WITH_AES_256_CBC_SHA256

• TLS_DH_DSS_WITH_AES_128_CBC_SHA256

• TLS_DH_RSA_WITH_AES_128_CBC_SHA256

• TLS_DHE_DSS_WITH_AES_128_CBC_SHA256

• TLS_RSA_WITH_CAMELLIA_128_CBC_SHA

• TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA

• TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA

• TLS_DHE_DSS_WITH_CAMELLIA_128_CBC_SHA

• TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA

• TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA

• TLS_DHE_RSA_WITH_AES_128_CBC_SHA256

• TLS_DH_DSS_WITH_AES_256_CBC_SHA256

• TLS_DH_RSA_WITH_AES_256_CBC_SHA256

• TLS_DHE_DSS_WITH_AES_256_CBC_SHA256

• TLS_DHE_RSA_WITH_AES_256_CBC_SHA256

• TLS_DH_anon_WITH_AES_128_CBC_SHA256

• TLS_DH_anon_WITH_AES_256_CBC_SHA256

• TLS_RSA_WITH_CAMELLIA_256_CBC_SHA

• TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA

• TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA

• TLS_DHE_DSS_WITH_CAMELLIA_256_CBC_SHA

• TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA

• TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA

• TLS_PSK_WITH_RC4_128_SHA

• TLS_PSK_WITH_3DES_EDE_CBC_SHA

• TLS_PSK_WITH_AES_128_CBC_SHA



• TLS_PSK_WITH_AES_256_CBC_SHA

• TLS_DHE_PSK_WITH_RC4_128_SHA

• TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA

• TLS_DHE_PSK_WITH_AES_128_CBC_SHA

• TLS_DHE_PSK_WITH_AES_256_CBC_SHA

• TLS_RSA_PSK_WITH_RC4_128_SHA

• TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA

• TLS_RSA_PSK_WITH_AES_128_CBC_SHA

• TLS_RSA_PSK_WITH_AES_256_CBC_SHA

• TLS_RSA_WITH_SEED_CBC_SHA

• TLS_DH_DSS_WITH_SEED_CBC_SHA

• TLS_DH_RSA_WITH_SEED_CBC_SHA

• TLS_DHE_DSS_WITH_SEED_CBC_SHA

• TLS_DHE_RSA_WITH_SEED_CBC_SHA

• TLS_DH_anon_WITH_SEED_CBC_SHA

• TLS_RSA_WITH_AES_128_GCM_SHA256

• TLS_RSA_WITH_AES_256_GCM_SHA384

• TLS_DH_RSA_WITH_AES_128_GCM_SHA256

• TLS_DH_RSA_WITH_AES_256_GCM_SHA384

• TLS_DH_DSS_WITH_AES_128_GCM_SHA256

• TLS_DH_DSS_WITH_AES_256_GCM_SHA384

• TLS_DH_anon_WITH_AES_128_GCM_SHA256

• TLS_DH_anon_WITH_AES_256_GCM_SHA384

• TLS_PSK_WITH_AES_128_GCM_SHA256

• TLS_PSK_WITH_AES_256_GCM_SHA384

• TLS_RSA_PSK_WITH_AES_128_GCM_SHA256

• TLS_RSA_PSK_WITH_AES_256_GCM_SHA384

• TLS_PSK_WITH_AES_128_CBC_SHA256

• TLS_PSK_WITH_AES_256_CBC_SHA384

• TLS_PSK_WITH_NULL_SHA256

• TLS_PSK_WITH_NULL_SHA384

• TLS_DHE_PSK_WITH_AES_128_CBC_SHA256

• TLS_DHE_PSK_WITH_AES_256_CBC_SHA384

• TLS_DHE_PSK_WITH_NULL_SHA256

• TLS_DHE_PSK_WITH_NULL_SHA384

• TLS_RSA_PSK_WITH_AES_128_CBC_SHA256

• TLS_RSA_PSK_WITH_AES_256_CBC_SHA384

• TLS_RSA_PSK_WITH_NULL_SHA256

• TLS_RSA_PSK_WITH_NULL_SHA384

• TLS_RSA_WITH_CAMELLIA_128_CBC_SHA256

• TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA256

• TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA256

• TLS_DHE_DSS_WITH_CAMELLIA_128_CBC_SHA256

• TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA256



• TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA256

• TLS_RSA_WITH_CAMELLIA_256_CBC_SHA256

• TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA256

• TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA256

• TLS_DHE_DSS_WITH_CAMELLIA_256_CBC_SHA256

• TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA256

• TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA256

• TLS_EMPTY_RENEGOTIATION_INFO_SCSV

• TLS_ECDH_ECDSA_WITH_NULL_SHA

• TLS_ECDH_ECDSA_WITH_RC4_128_SHA

• TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA

• TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA

• TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA

• TLS_ECDHE_ECDSA_WITH_NULL_SHA

• TLS_ECDHE_ECDSA_WITH_RC4_128_SHA

• TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA

• TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA

• TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA

• TLS_ECDH_RSA_WITH_NULL_SHA

• TLS_ECDH_RSA_WITH_RC4_128_SHA

• TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA

• TLS_ECDH_RSA_WITH_AES_128_CBC_SHA

• TLS_ECDH_RSA_WITH_AES_256_CBC_SHA

• TLS_ECDHE_RSA_WITH_NULL_SHA

• TLS_ECDHE_RSA_WITH_RC4_128_SHA

• TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA

• TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA

• TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA

• TLS_ECDH_anon_WITH_NULL_SHA

• TLS_ECDH_anon_WITH_RC4_128_SHA

• TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA

• TLS_ECDH_anon_WITH_AES_128_CBC_SHA

• TLS_ECDH_anon_WITH_AES_256_CBC_SHA

• TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA

• TLS_SRP_SHA_RSA_WITH_3DES_EDE_CBC_SHA

• TLS_SRP_SHA_DSS_WITH_3DES_EDE_CBC_SHA

• TLS_SRP_SHA_WITH_AES_128_CBC_SHA

• TLS_SRP_SHA_RSA_WITH_AES_128_CBC_SHA

• TLS_SRP_SHA_DSS_WITH_AES_128_CBC_SHA

• TLS_SRP_SHA_WITH_AES_256_CBC_SHA

• TLS_SRP_SHA_RSA_WITH_AES_256_CBC_SHA

• TLS_SRP_SHA_DSS_WITH_AES_256_CBC_SHA

• TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256

• TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384



• TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256

• TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384

• TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256

• TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384

• TLS_ECDH_RSA_WITH_AES_128_CBC_SHA256

• TLS_ECDH_RSA_WITH_AES_256_CBC_SHA384

• TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256

• TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384

• TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256

• TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384

• TLS_ECDHE_PSK_WITH_RC4_128_SHA

• TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA

• TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA

• TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA

• TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256

• TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384

• TLS_ECDHE_PSK_WITH_NULL_SHA

• TLS_ECDHE_PSK_WITH_NULL_SHA256

• TLS_ECDHE_PSK_WITH_NULL_SHA384

• TLS_RSA_WITH_ARIA_128_CBC_SHA256

• TLS_RSA_WITH_ARIA_256_CBC_SHA384

• TLS_DH_DSS_WITH_ARIA_128_CBC_SHA256

• TLS_DH_DSS_WITH_ARIA_256_CBC_SHA384

• TLS_DH_RSA_WITH_ARIA_128_CBC_SHA256

• TLS_DH_RSA_WITH_ARIA_256_CBC_SHA384

• TLS_DHE_DSS_WITH_ARIA_128_CBC_SHA256

• TLS_DHE_DSS_WITH_ARIA_256_CBC_SHA384

• TLS_DHE_RSA_WITH_ARIA_128_CBC_SHA256

• TLS_DHE_RSA_WITH_ARIA_256_CBC_SHA384

• TLS_DH_anon_WITH_ARIA_128_CBC_SHA256

• TLS_DH_anon_WITH_ARIA_256_CBC_SHA384

• TLS_ECDHE_ECDSA_WITH_ARIA_128_CBC_SHA256

• TLS_ECDHE_ECDSA_WITH_ARIA_256_CBC_SHA384

• TLS_ECDH_ECDSA_WITH_ARIA_128_CBC_SHA256

• TLS_ECDH_ECDSA_WITH_ARIA_256_CBC_SHA384

• TLS_ECDHE_RSA_WITH_ARIA_128_CBC_SHA256

• TLS_ECDHE_RSA_WITH_ARIA_256_CBC_SHA384

• TLS_ECDH_RSA_WITH_ARIA_128_CBC_SHA256

• TLS_ECDH_RSA_WITH_ARIA_256_CBC_SHA384

• TLS_RSA_WITH_ARIA_128_GCM_SHA256

• TLS_RSA_WITH_ARIA_256_GCM_SHA384

• TLS_DH_RSA_WITH_ARIA_128_GCM_SHA256

• TLS_DH_RSA_WITH_ARIA_256_GCM_SHA384

• TLS_DH_DSS_WITH_ARIA_128_GCM_SHA256



• TLS_DH_DSS_WITH_ARIA_256_GCM_SHA384

• TLS_DH_anon_WITH_ARIA_128_GCM_SHA256

• TLS_DH_anon_WITH_ARIA_256_GCM_SHA384

• TLS_ECDH_ECDSA_WITH_ARIA_128_GCM_SHA256

• TLS_ECDH_ECDSA_WITH_ARIA_256_GCM_SHA384

• TLS_ECDH_RSA_WITH_ARIA_128_GCM_SHA256

• TLS_ECDH_RSA_WITH_ARIA_256_GCM_SHA384

• TLS_PSK_WITH_ARIA_128_CBC_SHA256

• TLS_PSK_WITH_ARIA_256_CBC_SHA384

• TLS_DHE_PSK_WITH_ARIA_128_CBC_SHA256

• TLS_DHE_PSK_WITH_ARIA_256_CBC_SHA384

• TLS_RSA_PSK_WITH_ARIA_128_CBC_SHA256

• TLS_RSA_PSK_WITH_ARIA_256_CBC_SHA384

• TLS_PSK_WITH_ARIA_128_GCM_SHA256

• TLS_PSK_WITH_ARIA_256_GCM_SHA384

• TLS_RSA_PSK_WITH_ARIA_128_GCM_SHA256

• TLS_RSA_PSK_WITH_ARIA_256_GCM_SHA384

• TLS_ECDHE_PSK_WITH_ARIA_128_CBC_SHA256

• TLS_ECDHE_PSK_WITH_ARIA_256_CBC_SHA384

• TLS_ECDHE_ECDSA_WITH_CAMELLIA_128_CBC_SHA256

• TLS_ECDHE_ECDSA_WITH_CAMELLIA_256_CBC_SHA384

• TLS_ECDH_ECDSA_WITH_CAMELLIA_128_CBC_SHA256

• TLS_ECDH_ECDSA_WITH_CAMELLIA_256_CBC_SHA384

• TLS_ECDHE_RSA_WITH_CAMELLIA_128_CBC_SHA256

• TLS_ECDHE_RSA_WITH_CAMELLIA_256_CBC_SHA384

• TLS_ECDH_RSA_WITH_CAMELLIA_128_CBC_SHA256

• TLS_ECDH_RSA_WITH_CAMELLIA_256_CBC_SHA384

• TLS_RSA_WITH_CAMELLIA_128_GCM_SHA256

• TLS_RSA_WITH_CAMELLIA_256_GCM_SHA384

• TLS_DH_RSA_WITH_CAMELLIA_128_GCM_SHA256

• TLS_DH_RSA_WITH_CAMELLIA_256_GCM_SHA384

• TLS_DH_DSS_WITH_CAMELLIA_128_GCM_SHA256

• TLS_DH_DSS_WITH_CAMELLIA_256_GCM_SHA384

• TLS_DH_anon_WITH_CAMELLIA_128_GCM_SHA256

• TLS_DH_anon_WITH_CAMELLIA_256_GCM_SHA384

• TLS_ECDH_ECDSA_WITH_CAMELLIA_128_GCM_SHA256

• TLS_ECDH_ECDSA_WITH_CAMELLIA_256_GCM_SHA384

• TLS_ECDH_RSA_WITH_CAMELLIA_128_GCM_SHA256

• TLS_ECDH_RSA_WITH_CAMELLIA_256_GCM_SHA384

• TLS_PSK_WITH_CAMELLIA_128_GCM_SHA256

• TLS_PSK_WITH_CAMELLIA_256_GCM_SHA384

• TLS_RSA_PSK_WITH_CAMELLIA_128_GCM_SHA256

• TLS_RSA_PSK_WITH_CAMELLIA_256_GCM_SHA384

• TLS_PSK_WITH_CAMELLIA_128_CBC_SHA256



• TLS_PSK_WITH_CAMELLIA_256_CBC_SHA384

• TLS_DHE_PSK_WITH_CAMELLIA_128_CBC_SHA256

• TLS_DHE_PSK_WITH_CAMELLIA_256_CBC_SHA384

• TLS_RSA_PSK_WITH_CAMELLIA_128_CBC_SHA256

• TLS_RSA_PSK_WITH_CAMELLIA_256_CBC_SHA384

• TLS_ECDHE_PSK_WITH_CAMELLIA_128_CBC_SHA256

• TLS_ECDHE_PSK_WITH_CAMELLIA_256_CBC_SHA384

• TLS_RSA_WITH_AES_128_CCM

• TLS_RSA_WITH_AES_256_CCM

• TLS_RSA_WITH_AES_128_CCM_8

• TLS_RSA_WITH_AES_256_CCM_8

• TLS_PSK_WITH_AES_128_CCM

• TLS_PSK_WITH_AES_256_CCM

• TLS_PSK_WITH_AES_128_CCM_8

• TLS_PSK_WITH_AES_256_CCM_8

Note: This list was assembled from the set of registered TLS cipher suites at the time of writing. This listincludes those cipher suites that do not offer an ephemeral key exchange and those that are based on theTLS null, stream, or block cipher type (as defined in Section 6.2.3 of [TLS12]). Additional cipher suiteswith these properties could be defined; these would not be explicitly prohibited.



Acknowledgements

This document includes substantial input from the following individuals:

• Adam Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa Wilk, Costin Manolache,William Chan, Vitaliy Lvin, Joe Chan, Adam Barth, Ryan Hamilton, Gavin Peters, Kent Alstad, KevinLindsay, Paul Amer, Fan Yang, and Jonathan Leighton (SPDY contributors).

• Gabriel Montenegro and Willy Tarreau (Upgrade mechanism).

• William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro, Jitu Padhye, Roberto Peon, and RobTrace (Flow control).

• Mike Bishop (Extensibility).

• Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner, Mike Bishop, and Herve Ruellan (Substantialeditorial contributions).

• Kari Hurtta, Tatsuhiro Tsujikawa, Greg Wilkins, Poul-Henning Kamp, and Jonathan Thackray.

• Alexey Melnikov, who was an editor of this document in 2013.

A substantial proportion of Martin's contribution was supported by Microsoft during his employment there.

The Japanese HTTP/2 community provided invaluable contributions, including a number of implementationsas well as numerous technical and editorial contributions.


Authors' Addresses

Mike BelsheBitGoEmail: [email protected]

Roberto PeonGoogle, IncEmail: [email protected]

Martin Thomson (editor)Mozilla331 E Evelyn StreetMountain View, CA 94041United StatesEmail: [email protected]